feat: Split statistics components computation & output

[MelReyCG]

Issue: #3822
On top of PR : #3829

Planned refactor: The goal is to have a way to query statistics for solver (reservoir reference pressure for wells), with a clear separation with output statistics component.

The solution proposed here aims at giving a single data storage for each statistics component, and also aims at avoiding to recompute the same statistics for a given timestep.

To achieve these goals, I separated each roles, instead of having everything in each *PhaseStatistics class:

• RegionStatistics classes for resulting data storages at different levels:
  • sub-regions mpi-local kernel compute read-backs,
  • (cell element) regions,
  • solver discretisation MeshLevel,
• StatsAggregator classes to compute & reduce the statistics to rank 0, can be instanciated as solver helpers (as done for wells in the PR),
• StatsTask Group, the user component to schedule flow statistics computation & output to log & CSV.

Old stats data architecture (stats are sticked to regions, and shared by all components):

Problem : ProblemManager
|-> domain : DomainPartition
    |-> MeshBodies : Group
        |-> cartesianMesh : MeshBody
            |-> meshLevels : Group
                |-> Level0 : MeshLevel
                |   |-> nodeManager : NodeManager
                |   |   |  * RegionStatistics (aggregate, mpi reduced)  <- stats data
                |   |   |-> sets : Group
                |   |       | * all : Wrapper< index array >
                |   |       | * xneg : Wrapper< index array >
                |   |       [...] (other element sets)
                |   |
                |   |-> ElementRegions : ElementRegionManager
                |   |   |  * RegionStatistics (aggregate, mpi reduced)  <- stats data
                |   |   |-> Channel : CellElementRegion
                |   |   |   |-> cb-0_0_0 : CellElementSubRegion
                |   |   |   |   | * pressure : Wrapper< real64 array >
                |   |   |   |   | * temperature : Wrapper< real64 array >
                |   |   |   |   | * RegionStatistics (compute read-back)  <- stats data
                |   |   |   |   [...] (other fields)
                |   |   |   |
                |   |   |   |-> cb-0_0_1 : CellElementSubRegion
                |   |   |   |   | * pressure : Wrapper< real64 array >
                |   |   |   |   | * temperature : Wrapper< real64 array >
                |   |   |   |   | * RegionStatistics (compute read-back)  <- stats data
                |   |   |   |   [...] (other fields)
                |   |   |   |
                |   |   |   [...] (other sub-regions)
                |   |   |
                |   |   |-> Barrier : CellElementRegion
                |   |       |-> cb-1_0_0 : CellElementSubRegion
                |   |       |-> cb-1_0_1 : CellElementSubRegion
                |   |       |-> * RegionStatistics (compute read-back)  <- stats data
                |   |       [...] (other sub-regions)
                |   |
                |   [...] (other element managers)
                |
                [...] (other discretizations)

Proposed new way, with a clear storage for each aggregator instance:

Problem : ProblemManager
|-> domain : DomainPartition
    |-> MeshBodies : Group
        |-> cartesianMesh : MeshBody
            |-> meshLevels : Group
                |-> Level0 : MeshLevel
                |   |-> nodeManager : NodeManager
                |   |   |-> sets : Group
                |   |       | * all : Wrapper< index array >
                |   |       | * xneg : Wrapper< index array >
                |   |       [...] (other element sets)
                |   |
                |   |-> ElementRegions : ElementRegionManager
                |   |   |-> Channel : CellElementRegion
                |   |   |   |-> cb-0_0_0 : CellElementSubRegion
                |   |   |   |   | * pressure : Wrapper< real64 array >
                |   |   |   |   | * temperature : Wrapper< real64 array >
                |   |   |   |   [...] (other fields)
                |   |   |   |
                |   |   |   |-> cb-0_0_1 : CellElementSubRegion
                |   |   |   |   | * pressure : Wrapper< real64 array >
                |   |   |   |   | * temperature : Wrapper< real64 array >
                |   |   |   |   [...] (other fields)
                |   |   |   |
                |   |   |   [...] (other sub-regions)
                |   |   |
                |   |   |-> Barrier : CellElementRegion
                |   |       |-> cb-1_0_0 : CellElementSubRegion
                |   |       |-> cb-1_0_1 : CellElementSubRegion
                |   |       [...] (other sub-regions)
                |   |
                |   [...] (other element managers)
         ____   |   |
         |      |   |-> statistics : Group (storage for all stats)
         |      |       |-> flowStats : Group (storage for this instance stats)
         |      |       |   |-> regionsStatistics : RegionStatistics (aggregate)
         |      |       |       |-> Channel : RegionStatistics (aggregate, mpi reduced)
         |      |       |       |   |-> cb-0_0_0 : RegionStatistics (compute read-back)
 stats   |      |       |       |   |-> cb-0_0_1 : RegionStatistics (compute read-back)
 data -> |      |       |       |   [...] (other sub-regions stats)
         |      |       |       |
         |      |       |       |-> Barrier : RegionStatistics (aggregate, mpi reduced)
         |      |       |           |-> cb-1_0_0 : RegionStatistics (compute read-back)
         |      |       |           |-> cb-1_0_1 : RegionStatistics (compute read-back)
         |      |       |           [...] (other sub-regions stats)
         |      |       |
         |___   |       [...] (other stats storages)
                |
                [...] (other discretizations)

Decoupling the storage of the CellElementRegion will help to provide a way of obtaining statistics from arbitrary mesh regions (defined by sets for instance).

---

PR also propose to consolidate testFlowStatistics by also checking the total mass balance of the compositional cases.