Steiner tree algorithm support (PAAL)

[brettviren]

PAAL is needed for steiner tree. We have an unused copy in patrec but:

• it is needed in clus/
• only a few functions are needed.

Look at WCP's use of PAAL to see if it is feasible to include just the few functions actually needed. Otherwise, move patrec/inc/WireCellPatRec/paal* to clus.

[brettviren]

WCP apparently only needs these bits:

  struct less{};
  class nearest_recorder;
  nearest_recorder make_nearest_recorder();

They have been interned into clus/src/PAAL.h all in namespace WireCell::Clus::PAAL. The WCP code has been translated to WCT in SteinerGrapher_fake.cxx. It compiles but I don't know enough about what it actually does to provide code that actually exercises it.

This is pushed in commit: 81fa291

[brettviren]

Actually, reopen this until we add the steiner code stuff into GraphAlgorithms. It relies on Dijkstra's shortest paths and so should be entombed as a GA in a similar way. I don't yet understand it well enough to do this.

[brettviren]

This algorithm actually has the name: "graph voronoi tessellation". It is now moved into clus' Graphs as voronoi(). It's uncached as it would need to be cached with a key hashed on the set of terminals. This doesn't seem appropriate. So, caller should handle whatever caching/reuse that may be useful.

Commit: 0d98921

[brettviren]

voronoi is just first step. Now there is also steiner_graph(). Here is the example from the doctest. First a made up graph:

And the steiner graph connecting terminals (0, 1 and 2) through shortest paths through the vertices. Vertices 3 and 4 are still included and without edges so that the graph retains the same indexing as the original.