rapidpe_ced: define default behavior for some options

Sinead Walsh identified that a few options in the DAG writer were getting handled improperly by their default values. This overrides that behavior for two: --coinc-xml and --fmax. The Fmax fix is to give it a default of 2048 if it's not specified (only for the ILE sub writer) and the coinc XML fix (more hacky) is to set the option to False if it doesn't appear in the command line to the DAG writer. This is hacky because the sub writer in dagutils.py only ignores and option given as a kwarg if its set to False. This whole bit of code needs a rewrite, but this should bandage the immediate problem.