Corrected bugs in the distance_array functions of lm/gl/hp space and in HPLMTransformation and LMHPTransformation

- Remove mmax. Now, lmax == mmax
- Create index and l-number on the fly

- Cython extension to return the correct l-number for an index

- Move kernel construction to after transform
- Use d2o.arange for distance_array method in HPSpace and GLSpace

-> 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.

- Refactor _smooth_helper
- Remove inplace property
- Rename method compute_k_array to distance_array in RGSpace
- Fix compute_k_array usage