Add the option to extract more sophisticated meshes when using Gmsh

[sblauth]

Closes #281

[codecov]

Codecov Report

Merging #282 (6e631c6) into main (ff0efab) will decrease coverage by 0.04%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
- Coverage   92.93%   92.89%   -0.04%     
==========================================
  Files          84       84              
  Lines        6518     6522       +4     
==========================================
+ Hits         6057     6058       +1     
- Misses        461      464       +3     

Files Changed	Coverage Δ
cashocs/_cli/_extract_mesh.py	30.43% <0.00%> (-2.90%)	⬇️
cashocs/io/mesh.py	93.72% <92.31%> (-0.46%)	⬇️

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