Custom likelihood feature

Added feature to provide bilby-pipe with a .py script containing a custom likelihood.

Refactoring

refactored parser.py into several functions based on the parser argument groups

Added a few FIXME (described below) that I would like to discuss with the bilby-pipe developers before working on.

BilbyArgParser: has several if statements that can be merged to one line to improve readability
BilbyArgParser: parse_known_args has a lot of code in it, which could be broken into several smaller functions to make a little easier to read+test
MainInput: too much code in one file, making it difficult to follow. Suspect that it could be broken into a few smaller classes
create_main_parser: this is a function which just calls another function... unnecessary wrapping since create_main_parser does no real work other than calling another function.
parse_args: this function might belong in parser.py rather than in utils.py

Edited May 30, 2019 by Avi Vajpeyi