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

- 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.

- 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.

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.