Convert to scipy.sparse.csc_matrix for PyPardiso

[loganbvh]

See #74

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.33%. Comparing base (ac8b2d9) to head (776365d).

Files	Patch %	Lines
tdgl/finite_volume/operators.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   86.35%   86.33%   -0.02%     
==========================================
  Files          40       40              
  Lines        4491     4492       +1     
==========================================
  Hits         3878     3878              
- Misses        613      614       +1     

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