adds metadata to output

[jmbhughes]

PR summary

• Adds metadata to output
• Improves documentation about units

Todos

• Finish metadata propagation
• Make sure everything always passes as an ndcube
• Test the metadata propagation

Test plan

Actually write some unit tests

Breaking changes

None

Related Issues

• closes Document weights units as 1/sigma #58
• closes Write at least a few tests #3
• closes Improve file output format #33

[codecov]

Codecov Report

Attention: Patch coverage is 84.90566% with 8 lines in your changes missing coverage. Please review.

Project coverage is 82.92%. Comparing base (c6ae31a) to head (7f015cc).

Files	Patch %	Lines
overlappogram/io.py	74.19%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #59       +/-   ##
==========================================
+ Coverage   0.00%   82.92%   +82.92%     
==========================================
  Files          7        7               
  Lines        370      410       +40     
  Branches      61       71       +10     
==========================================
+ Hits           0      340      +340     
+ Misses       370       54      -316     
- Partials       0       16       +16     

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