m-to-n restart (new)

[MTCam]

Overrides #973 to add m-to-n restart capabilities

Adds:

• bin/meshdist.py: utility for partitioning gmsh-format meshes into rank-specific pkl files, and creates some partition mapping files needed by bin/redist.py
• bin/redist.py: reads mirgecom restart files and mapping files created by meshdist.py to create a new restart dataset for a different partitioning.
• bin/mapdecomp.py: Diagnostic tool that discovers a bunch of stuff about a given partitioning, like how many rank/part neighbors each rank/part has
• some documentation

Modifes:

• simutil.py: Adds pkl file-based partitioning, arbitrary M reader ranks - to - N writer ranks partitioning (like node-local partitioning)
• restart.py: Adds utils for mapping data from m-to-n partitionings, and m-to-n restart
• test_resetart.py: Adds some tests for addded utils (with required data)

Questions for the review:

• Is the scope and purpose of the PR clear?
  • The PR should have a description.
  • The PR should have a guide if needed (e.g., an ordering).
• Is every top-level method and class documented? Are things that should be documented actually so?
• Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• Does the implementation do what the docstring claims?
• Is everything that is implemented covered by tests?
• Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[anderson2981]

Excellent. Nice documentation