Add new function cashocs.io.extract_mesh_from_xdmf and corresponding CLI

[sblauth]

This PR adds a new function cashocs.io.extract_mesh_from_xdmf which can be used to extract a mesh out of an XDMF state file (such as the ones written by cashocs).
Additionally, this also has the command line interface cashocs-extract_mesh.

Closes #268

[codecov]

Codecov Report

Merging #277 (0683ef9) into main (8f3653a) will increase coverage by 0.35%.
The diff coverage is 73.08%.

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
+ Coverage   92.59%   92.93%   +0.35%     
==========================================
  Files          83       84       +1     
  Lines        6460     6507      +47     
==========================================
+ Hits         5981     6047      +66     
+ Misses        479      460      -19     

Files Changed	Coverage Δ
cashocs/_cli/_extract_mesh.py	33.33% <33.33%> (ø)
cashocs/_cli/__init__.py	100.00% <100.00%> (ø)
cashocs/_cli/_convert.py	91.11% <100.00%> (+2.88%)	⬆️
cashocs/_utils/__init__.py	100.00% <100.00%> (ø)
cashocs/_utils/helpers.py	98.15% <100.00%> (+0.07%)	⬆️
cashocs/io/__init__.py	100.00% <100.00%> (ø)
cashocs/io/mesh.py	94.38% <100.00%> (+19.38%)	⬆️

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