Add support for parsing default GWF api from GWPY_FRAME_LIBRARY environment variable

[duncanmmacleod]

This PR adds support for parsing the preferred (default) GWF API from the GWPY_FRAME_LIBRARY environment variables, allowing users to override the default search order.

Closes #1720.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ffffb86) 94.6% compared to head (7c4ba05) 94.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1726     +/-   ##
=======================================
- Coverage   94.6%   94.3%   -0.3%     
=======================================
  Files        241     242      +1     
  Lines      18620   18642     +22     
=======================================
- Hits       17619   17598     -21     
- Misses      1001    1044     +43     

Flag	Coverage Δ
Conda	94.3% <100.0%> (+<0.1%)	⬆️
Linux	94.2% <100.0%> (-0.4%)	⬇️
Windows	85.0% <100.0%> (+<0.1%)	⬆️
macOS	93.9% <100.0%> (+<0.1%)	⬆️
python3.10	94.3% <100.0%> (+0.2%)	⬆️
python3.11	94.3% <100.0%> (+0.1%)	⬆️
python3.12	?
python3.8	94.3% <100.0%> (-0.3%)	⬇️
python3.9	94.3% <100.0%> (+<0.1%)	⬆️

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