plug in InitUserMeshBlockData

[Yurlungur]

PR Summary

As discussed in the call today and on the Matrix channel, there's a need for initializing variables and setting grid quantities once per re-meshing, but not more frequently than that. To enable this, I re-enable InitUserMeshBlockData, which is set in the application input object. This is now called at the beginning of Mesh::Initialize before anything else. Mesh::Initialize is called at the beginning of a simulation and upon each remeshing. This means data set in this function is expected to remain constant except when remeshing, and is available to problem generators, boundary conditions, and FillDerived.

I also hook it in to the calculate_pi example to show it works.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• CI has been triggered on Darwin for performance regression tests.
• (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

@par-hermes format

[Yurlungur]

I have checked that this fits our needs downstream.

[brryan]

Looks good to me. Only comment is would it be worth mentioning this usage of InitUserMeshBlockData in e.g. ~/github/parthenon/docs/amr.md? From the existing documentation it's not immediately apparent (to me at least) that InitUserMeshBlockData is called after every remeshing step.

[Yurlungur]

Oh good call, @brryan . Yes documentation should be added.

[jlippuner]

Looks great! I just have a small suggestion to rename the function to avoid potential confusion with MeshBlockData, but I'll defer to your judgement.