Feature/pathlib support

[sblauth]

Add pathlib support and remove os.path usage

Closes #102

[codecov-commenter]

Codecov Report

Merging #103 (0f13818) into main (2c4a3fd) will increase coverage by 0.01%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   94.40%   94.40%   +0.01%     
==========================================
  Files          65       65              
  Lines        5533     5538       +5     
==========================================
+ Hits         5223     5228       +5     
  Misses        310      310              

Impacted Files	Coverage Δ
cashocs/geometry/mesh_handler.py	93.15% <87.50%> (+0.06%)	⬆️
...n/shape_optimization/shape_optimization_problem.py	96.95% <100.00%> (ø)
cashocs/geometry/mesh.py	100.00% <100.00%> (ø)
cashocs/io/output.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more