- 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

Adapted imports and setup.py to changes.

- rename transforms submodule to transformations
- Add a Transformation class which wraps the actual transform class
- Derive and implement RGRGTransformation
- Add tests

- Merge _get_transform_override and _get_transform
- Move check_codomain for rg_space to Transform as a staticmethod
- Add tests for transforms

Added functionality for multiple field_types per field.

Condensed functional structure in nifty_fft.py.

- Refactor based on discussion on 08.06
- Add test for FFTW transform