- Cython extension to return the correct l-number for an index

- Move kernel construction to after transform
- Use d2o.arange for distance_array method in HPSpace and GLSpace

Started to consolidate the code base of LM <-> GL/HP transformations.

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

Fixed furthermore:
- Let's undo the changes in the RGSpace.__init__ (see the comment on gitlab)
- The HPSpaceParadict still has `distances` in the list.
- HPSpace.copy: Is this method really necessary? The one from Space should be sufficient, right?
- HPSpace.total_volume is missing
- Unnecessary imports of `random`, `LMSpace`,... in hp_space.py
- I removed the plotting functionality from the space classes.
- GLSpace.copy should also not be necessary
- GLSpace.dim is missing
- GLSpace.vol should be called `total_volume`. Also its implementation is defect: there is no self.distances for GLSpace. The total volume is just 4*pi.
- Use nifty_utilities.cast_axis_to_tuple in order to sanitize `axes` input in GLSpace.weight
- typo in `new_shape = np.ones(x.shape)`

- Remove distances from HPSpace paradict
- Cleanup GLSpace and add weight method

Adapted imports and setup.py to changes.