Add support for spin polarization to Python and C API

[awvwgk]

• support creation of spin polarization object in API
• track number of spin channels in calculator object for safe wavefunction reset
• move calculator_push_back export from calculator.h to container.h to avoid cyclic imports
• create interaction containers in Python API via .add

[codecov]

Codecov Report

Merging #117 (4cdea77) into main (c21f9df) will decrease coverage by 0.37%.
The diff coverage is 58.92%.

❗ Current head 4cdea77 differs from pull request most recent head 0853890. Consider uploading reports for the commit 0853890 to get more accurate results

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
- Coverage   72.49%   72.12%   -0.37%     
==========================================
  Files         125      121       -4     
  Lines       15556    15210     -346     
  Branches     4643     4658      +15     
==========================================
- Hits        11277    10970     -307     
+ Misses       2198     2147      -51     
- Partials     2081     2093      +12     

Files Changed	Coverage Δ
src/tblite/api/container.f90	49.23% <56.60%> (+49.23%)	⬆️
src/tblite/api/calculator.f90	65.74% <100.00%> (+2.92%)	⬆️

... and 4 files with indirect coverage changes

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