Start to refactor out a traject Registration object to stop passing
around so many parameters.
More use for Registration object in path.py.
parameter_factory is always made in Registration object.
Factor out combining registrations into Registration object, simplifying
register_subpath.
implement get_registration to hopefully let it be reusable between
register_path and register_subpath in the future.
Reorganize get_registration to follow order of parameters better.
Refactor register_subpath to use get_registration too.