support **kwargs in gemmification decoratored fn

[kmdalton]

This PR modifies the @spacegroupify and @cellify decorators so as to support decorating functions which take arbitrary keyword arguments (**kwargs). Specifically, it alters the usage of the BoundArguments instance in the decorated function call.

f(**bargs.arguments) becomes f(*bargs.args, **bargs.kwargs)

[JBGreisman]

This looks good -- makes sense to update these decorators to fully support arbitrary keyword arguments