C deprecation

[damonge]

Deprecates much of the C-layer code, exposing it to python. Now:

• All read/write of workspaces happens in python
• Only the building blocks of the mode-coupling coefficients are calculated in C and then passed to python to assemble the full mode-coupling matrices. This more modular approach will make it easier to use other Wigner 3J libraries (e.g. ducc or threej_cosmo).

Future extensions of the code should be significantly easier to maintain.

Unfortunately this changes some minor aspects of the API, which will eventually trigger a new major version of the code. Will wait a bit before making a new release, however.