- Use attrs for storing metadata
- Remove pickle usage from Field and PowerSpace

Fixed several bugs.

Removed NIFTy 2 files that have not been ported so far.

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

- Refactor _smooth_helper
- Remove inplace property
- Rename method compute_k_array to distance_array in RGSpace
- Fix compute_k_array usage

Modified power_analyze in Field for complex power spectra.

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 smooth from abstract Space class
- Remove smooth from RGSpace
- Remove smooth and transform from HPSpace
- Move all HPSpace specific attributes to paradict

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.