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.

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

- 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.

- 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.

Added functionality for multiple field_types per field.

Therefore made the Field_type base class more advanced.
Added default field dtype to nifty configuration.