Migrate LIGO_LW handling to use python-ligo-lw

[duncanmmacleod]

This PR migrates the gwsum.triggers module to use python-ligo-lw for handling LIGO_LW files, instead of glue.ligolw.

All of the actual reading is done by GWpy, so this change doesn't actually change how the files are read, but instead modifies some edge cases (e.g. empty table creation, etc).

[codecov]

Codecov Report

Merging #331 (d7536a6) into master (bf7f1ea) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #331   +/-   ##
=======================================
  Coverage   50.31%   50.31%           
=======================================
  Files          60       60           
  Lines        8658     8658           
=======================================
  Hits         4356     4356           
  Misses       4302     4302           

Flag	Coverage Δ
Linux	50.31% <100.00%> (ø)
macOS	50.31% <100.00%> (ø)
python3.10	50.31% <100.00%> (ø)
python3.7	49.51% <100.00%> (ø)
python3.8	50.31% <100.00%> (ø)
python3.9	50.31% <100.00%> (ø)

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

Impacted Files	Coverage Δ
gwsumm/triggers.py	65.50% <100.00%> (ø)

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[eagoetz]

LGTM