Read mesh domains from vtk file 363

[ktbolt]

Finish #363.

I also cleaned up the interface for VtkData.h.

I don't think there are any tests for this.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.75%. Comparing base (8df8135) to head (26a622f).

Files with missing lines	Patch %	Lines
Code/Source/solver/VtkData.cpp	0.00%	26 Missing ⚠️
Code/Source/solver/vtk_xml.cpp	0.00%	17 Missing ⚠️
Code/Source/solver/read_msh.cpp	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #513      +/-   ##
==========================================
- Coverage   67.85%   67.75%   -0.10%     
==========================================
  Files         168      168              
  Lines       32749    32797      +48     
  Branches     5748     5766      +18     
==========================================
  Hits        22221    22221              
- Misses      10391    10439      +48     
  Partials      137      137              

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