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

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

Fixed a bug in fft_operator.
Fixed a inconsistency in GFFT interface.

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

Fixed violation of LSP for implemented keyword/property.

- Add get_default_codomain function in utilities
- Move all transforms to a subfolder inside the fft_operator
- Correct weight method calls in transform method of each transformation