Add doctests to CI and fix existing failures

[djhoese]

Also adds more strict pytest settings (previously just defaults)

Written by Claude. Scrutinized by me.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.58%. Comparing base (e74a0ef) to head (ec581f8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   89.04%   89.58%   +0.54%     
==========================================
  Files          20       20              
  Lines        1515     1517       +2     
==========================================
+ Hits         1349     1359      +10     
+ Misses        166      158       -8     

Flag	Coverage Δ
unittests	89.58% <100.00%> (+0.54%)	⬆️

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[coveralls]

coverage: 88.693% (+0.5%) from 88.176% — djhoese:bugfix-doctests into pytroll:main

[djhoese]

This is a pretty obvious win. Merging as I don't see a reason to waste maintainer time reviewing it.