Applications can inject their own MeshObject via an input file

[andrsd]

If needed, applications can override InputFile::create_mesh_object and
supply thier own mesh object if needed. This opens the design more.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.52%. Comparing base (8de117a) to head (5d878cc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #736   +/-   ##
=======================================
  Coverage   85.52%   85.52%           
=======================================
  Files         199      199           
  Lines       13470    13474    +4     
=======================================
+ Hits        11520    11524    +4     
  Misses       1950     1950           

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