Add 'orbit' unit for orbital period

[lpsinger]

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[codecov]

Codecov Report

Merging #43 (2c5365f) into main (bcc389a) will increase coverage by 0.88%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   24.64%   25.53%   +0.88%     
==========================================
  Files          26       29       +3     
  Lines        1424     1441      +17     
==========================================
+ Hits          351      368      +17     
  Misses       1073     1073              

Impacted Files	Coverage Δ
dorado/scheduling/units/__init__.py	100.00% <100.00%> (ø)
dorado/scheduling/units/equivalencies.py	100.00% <100.00%> (ø)
dorado/scheduling/units/orbital.py	100.00% <100.00%> (ø)

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