Add UnstructuredQuadMesh to load_mesh_serial

[andrewwinters5000]

Necessary structure to reconstruct the unstructured quadrilateral mesh for conversion to vtk via Trixi2Vtk

[codecov]

Codecov Report

Merging #575 (abc2b29) into main (ca0a78f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head abc2b29 differs from pull request most recent head 611d6e6. Consider uploading reports for the commit 611d6e6 to get more accurate results

@@           Coverage Diff           @@
##             main     #575   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files         138      138           
  Lines       13837    13840    +3     
=======================================
+ Hits        12969    12972    +3     
  Misses        868      868           

Flag	Coverage Δ
unittests	93.72% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/callbacks_step/save_restart_dg.jl	88.29% <100.00%> (-0.13%)	⬇️
src/mesh/mesh_io.jl	95.75% <100.00%> (+0.10%)	⬆️

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