Add missing docstrings and refactor a little

[mike0sv]

closes #70

[codecov]

Codecov Report

Merging #261 (5498531) into main (83f169f) will decrease coverage by 0.06%.
The diff coverage is 91.00%.

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
- Coverage   89.23%   89.17%   -0.07%     
==========================================
  Files          77       75       -2     
  Lines        5544     5568      +24     
==========================================
+ Hits         4947     4965      +18     
- Misses        597      603       +6     

Impacted Files	Coverage Δ
mlem/config.py	97.53% <ø> (ø)
mlem/core/artifacts.py	99.40% <ø> (ø)
mlem/core/hooks.py	95.08% <ø> (ø)
mlem/core/import_objects.py	97.56% <ø> (ø)
mlem/core/index.py	67.32% <ø> (ø)
mlem/core/meta_io.py	93.92% <ø> (ø)
mlem/core/model.py	93.38% <ø> (ø)
mlem/core/requirements.py	95.05% <ø> (ø)
mlem/ext.py	86.45% <ø> (ø)
mlem/runtime/interface.py	84.78% <ø> (ø)
... and 33 more

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[aguschin]

Wow. This is awesome!