GW-specific result object

In #230 (closed), @charlie.hoy suggested adding the approximant to the result object and that is a good idea. More generally, I can imagine there being a number of CBC-specific information we'd like to include and also GW-specific methods we'd like the result object to have (for example, a plot_skymap method).

Keeping with the 1st amendment, separation of core and gw, this will require a special "bilby.gw.GWResult" object to be created which inherits from bilby.core.Result, but adds the additional data to store and new methods.

This would make a good issue for someone to pursue if you want to get to grips with the internals of the bilby results classes.