Docstrings

This reverts commit 01082932.

- Fix serialization, total_volume and weight methods/properties in
PowerSpace
- Fix copy in LMSpace

and added plot method in all spaces, to be implemented

Signed-off-by: Mihai Baltac <mbaltac@mpa-garching.mpg.de>

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

Fixed several bugs.

Added keyword arguments mean and std to Field.power_synthesize. Bug fixes in Field, SmoothingOperator, LMSpace and Space.

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

Corrected bugs in the distance_array functions of lm/gl/hp space and in HPLMTransformation and LMHPTransformation

- Remove mmax. Now, lmax == mmax
- Create index and l-number on the fly

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

compute_k_array is still missing.

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

- Create skeleton for TransformationOperator
- Minor bug fixes in Field, FFTTransform and RGRGTransformations and
  __init__.py for endomorphic_operator