-> A Field on a PowerSpace can now be initialized with a spectral function.

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

compute_k_array is still missing.

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.

Moved unitary property to LinearOperator.

- 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

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