add docstrings

[cbyrohl]

Add docstrings to all functions. Also fixes #99 and #118.

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (4efcb95) 54.84% compared to head (e32bec6) 54.82%.

Files	Patch %	Lines
src/scida/misc.py	38.46%	8 Missing ⚠️
src/scida/fields.py	70.58%	3 Missing and 2 partials ⚠️
src/scida/convenience.py	0.00%	1 Missing ⚠️
src/scida/customs/arepo/dataset.py	80.00%	1 Missing ⚠️
src/scida/customs/swift/series.py	0.00%	1 Missing ⚠️
src/scida/interfaces/mixins/spatial.py	0.00%	1 Missing ⚠️
src/scida/series.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   54.84%   54.82%   -0.02%     
==========================================
  Files          63       63              
  Lines        5742     5742              
  Branches     1139     1140       +1     
==========================================
- Hits         3149     3148       -1     
  Misses       2353     2353              
- Partials      240      241       +1     

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