Add __version__ module attribute

@moritz.huebner @colm.talbot @paul-lasky @gregory.ashton Can a module-level __version__ attribute be added to bilby (see, e.g., this PEP)? This will mean that the version only needs to be updated in one place and can be imported from the package as required (e.g., currently the version number in the docs/conf.py is out-of-date).

I've made the changes here and can add a MR for this patch if people are happy with it going ahead.