Added support for submitting jobs to the Open Science Grid

This PR updates bilby_pipe to support running on the Open Science Grid. The highlights are

• the bilby_pipe command-line script now supports the --osg option
• updated options for vanilla-universe checkpointing (cc @james-clark)

To support the above, I chose to do a bit of restructuring in the relevant methods to clean up how the submit files are generated, including

• moved out/err/log file path options to a method
• moved checkpointing options to a method
• moved file transfer options to a method and fixed directory names for transfer_output_files
• track extra_lines as a list, not a monolithic str (this is natively supported by pycondor anyway)
• support appending requirements to self.requirements (required for OSG options)

Fixes #103 (closed).