Add convenience functions

[darbyjohnston]

Summarize your change.

Add a few convenience functions:

• Add a TimeRange constructor that takes three doubles; the start time, duration, and rate. This allows you to construct a TimeRange like: TimeRange(0.0, 48.0, 24.0), which is a bit more convenient than: TimeRange(RationalTime(0.0, 24.0), RationalTime(48.0, 24.0)).
• Add RationalTime::is_valid_time() to complement RationalTime::is_invalid_time(). This can be a bit easier to read: if (time.is_valid_time()), instead of: if (!time.is_invalid_time()).
• Add TimeRange::is_invalid_range() and TimeRange::is_valid_range() to match RationalTime.

Small fix:

• Remove explicit from TimeRange constructors. The explicit keyword only needs to be used with constructors that take a single argument.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.80%. Comparing base (c0e97b0) to head (235d812).
Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1825      +/-   ##
==========================================
+ Coverage   84.11%   84.80%   +0.68%     
==========================================
  Files         198      176      -22     
  Lines       22241    12742    -9499     
  Branches     4687     1181    -3506     
==========================================
- Hits        18709    10806    -7903     
+ Misses       2610     1758     -852     
+ Partials      922      178     -744     

Flag	Coverage Δ
py-unittests	84.80% <100.00%> (+0.68%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/opentime/rationalTime.h	95.83% <100.00%> (+1.15%)	⬆️
src/opentime/timeRange.h	98.30% <100.00%> (-0.79%)	⬇️
...lineio/opentime-bindings/opentime_rationalTime.cpp	94.87% <100.00%> (+0.27%)	⬆️
...imelineio/opentime-bindings/opentime_timeRange.cpp	98.07% <100.00%> (-1.93%)	⬇️
tests/test_opentime.py	99.75% <100.00%> (+<0.01%)	⬆️

... and 124 files with indirect coverage changes

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