Replacing uses of deprecated features in 'ligo.lw'

This should fix #27 (closed)

The module-level function ligo.lw.table.get_table was an alias for the class method ligo.lw.table.Table.get_table, and was deprecated recently. This replaces all calls with the class method.

There were also uses of the gz keyword argument to ligo.lw.utils.write_filename. gz=True has been replaced with compress="gz", and in all of our cases the default behavior of compress=None should automatically set this, so I have just removed the argument without replacing it.