Add Doxygen comments to RationalTime #2

[darbyjohnston]

Fixes #1753

Summarize your change.

Adds Doxygen comments to the RationalTime class. These comments are based on the Python doc strings with some editing for consistency.

(This PR replaces #1746 which got into some git trouble.)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.71%. Comparing base (c0e97b0) to head (9325b51).
Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1765      +/-   ##
==========================================
- Coverage   84.11%   81.71%   -2.41%     
==========================================
  Files         198      176      -22     
  Lines       22241    12315    -9926     
  Branches     4687     3020    -1667     
==========================================
- Hits        18709    10063    -8646     
+ Misses       2610     1713     -897     
+ Partials      922      539     -383     

Flag	Coverage Δ
py-unittests	81.71% <ø> (-2.41%)	⬇️

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Files with missing lines	Coverage Δ
src/opentime/rationalTime.h	92.55% <ø> (-2.13%)	⬇️

... and 65 files with indirect coverage changes

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