Fixed bug in appending to LIGO_LW file
This PR fixes #912 (closed) by correcting an error in the logic in gwpy.io.ligolw. I also improved the unit tests for that module by adding tests for write_tables() and write_tables_to_document().
This PR fixes #912 (closed) by correcting an error in the logic in gwpy.io.ligolw. I also improved the unit tests for that module by adding tests for write_tables() and write_tables_to_document().