Add documentation for union_bands and band_count creation options (for the online doc)

[danscales]

Add documentation for union_bands and band_count

Also, added descriptions for data_type and no_data.

I somewhat guessed on the meaning of band_count based on a few mentions in the code, so I definitely welcome comments on all of the descriptions, esp. union_bands and band_count.

[dmannarino]

Thanks for adding these!

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.56%. Comparing base (7e3a1a8) to head (6f5aec9).

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #694   +/-   ##
========================================
  Coverage    76.56%   76.56%           
========================================
  Files          143      143           
  Lines         6704     6704           
========================================
  Hits          5133     5133           
  Misses        1571     1571           

Flag	Coverage Δ
unittests	76.56% <100.00%> (ø)

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