Allow other projections than just Polar Stereographic

[mdtanker]

Enable plotting functions for grids of other projections than just EPSG 3031 and 3413. This will add an additional parameter epsg to all user-facing functions, so users can provide either the hemisphere or the EPSG code directly. For all private functions, all hemisphere parametes will be replaced with the epsg parameter. This shouldn't break anything, as hemisphere can be provided as in the past, but internally will be converted to the relevant EPSG code.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 13.28%. Comparing base (5c18f10) to head (48f048b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/polartoolkit/maps.py	0.00%	36 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
- Coverage   13.34%   13.28%   -0.06%     
==========================================
  Files           7        7              
  Lines        4010     4028      +18     
==========================================
  Hits          535      535              
- Misses       3475     3493      +18     

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[mdtanker]

Closed in favor of #324