Run ISSM over multiple input files (multiple ice sheets or glaciers)

[agstub]

This PR adds the capability to run ISSM over multiple input files (e.g., Greenland + Antarctica) intended for coupling with the GEOS Earth system model. This solution is based on the fact that ISSM simulations are encapsulated in FemModel objects. The main changes to the workflow in esmfbinders.cpp are as follows:

1. InitializeISSM: Given an experiment directory EXPDIR, count all of the input (.bin) files and initialize a FemModel object for each one. Return the number of elements and nodes across all models on each process.
2. GetElementsISSM and GetNodesISSM: Create IDs for the mesh elements and nodes that are unique across all models (input files / glaciers).
3. On the ESMF/GEOS side: Create a global ESMF_Mesh object that is the union of all ISSM meshes (i.e., Greenland + Antarctica + whatever else is in EXPDIR). This ESMF_Mesh object can be used for regridding imports/exports between the GCM and ISSM. For an example see examples/GEOSInput/main_esmf.f90
4. RunISSM: Given the time step and SMB forcing from the GCM, run a single time step of each model. Import/export pointers (defined on the global mesh) are split up appropriately across models and processes.

• Some example plots for Greenland and Antarctica are in this notebook.
• Note: Minor changes (simplifications actually) will be required in GEOSgcm_GridComp branch Add ISSM ice sheet model GridComp to Landice GEOSgcm_GridComp#1203

Just FYI: @lcandre2 @gmao-rreichle @weiyuan-jiang

[agstub]

works as expected in GEOS
(on discover, see: /discover/nobackup/agstubbl/GEOS/notebooks/plotting.ipynb)
will merge branch, create new module, update gridcomp branch