added all the docstrings for PowerSpace and changed PowerSpace.log to PowerSpace.logarithmic as well as PowerSpace.harmonic_domain to harmonic_partner

Reordered methods in space classes.
Made Space.weight to an abc.abstractmethod.

Adapted imports and setup.py to changes.

WIP: Cleaning up the structure of Space classes. Tidied up the chain PowerIndices -> PowerIndexFactory -> Space -> PowerSpace; including paradict.