Add to_csv/from_csv to Colormap

[djhoese]

This is needed by #99 so that a colormap can be fully reconstructed with minimal information. This is a fairly useful feature by itself so I thought I'd make a separate PR for it. This is not as flexible as the loading logic in Satpy for colormaps, but I'm not sure if it should be. It would require extra keyword arguments like colormap_mode, etc. Maybe it should be added?

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

Merging #101 (66d17af) into main (eb54137) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   91.84%   92.21%   +0.36%     
==========================================
  Files          10       10              
  Lines        3239     3389     +150     
==========================================
+ Hits         2975     3125     +150     
  Misses        264      264              

Flag	Coverage Δ
unittests	92.21% <100.00%> (+0.36%)	⬆️

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Impacted Files	Coverage Δ
trollimage/colormap.py	100.00% <100.00%> (ø)
trollimage/tests/test_colormap.py	100.00% <100.00%> (ø)

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[djhoese]

I've ported all of the "from file" style from satpy and included it in this PR.