Update to handle new LIGO_LW-format XML files

A new version of the LIGO_LW XML format is being pushed that replaces the bulky ilwd:char ID strings with simpler ID ints. It would be great to have this client be able to read/write the old format and the new concurrently as a precursor to updating GDS upstream to write files with the new ID strings.

The change would basically be to use ligo.lw instead of glue.ligolw in all places, but you would probably need to supply a compatibility flag (--ilwdchar-compat or similar) to allow users to select the old or new format.