Stp/mfem transfers

[seimtpow]

Enable copy transfers from one MFEM app to another MFEM app with identical meshes.

• Tests for from, to and between subapp cases
• Minor fixes to make multi-app system work
  • Add check that MFEM is not already initialised
  • Pass the moose MPI comm through to MFEM instead of MPI_COMM_WORLD
• Checks that mesh is 3d in MFEMMesh buildMesh function (can be removed when 2d meshes are supported)
• Changed platypus.mk to respect MFEM_DIR if already set
• Some minor syntax changes so clangd would stop moaning
  (https://github.com/orgs/aurora-multiphysics/projects/7?pane=issue&itemId=83597055)

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/transfers/MultiAppMFEMCopyTransfer.C	88.37%	5 Missing ⚠️
src/coefficients/MFEMCoefficient.C	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/executioners/MFEMExecutioner.C	100.00% <100.00%> (ø)
src/executioners/MFEMSteady.C	96.36% <100.00%> (+0.06%)	⬆️
src/mesh/MFEMMesh.C	97.77% <100.00%> (+0.05%)	⬆️
src/coefficients/MFEMCoefficient.C	80.00% <50.00%> (ø)
src/transfers/MultiAppMFEMCopyTransfer.C	88.37% <88.37%> (ø)