Uniform the API of the noise generation module

[ziotom78]

This PR introduces some breaking changes to the noise module. Specifically:

• There are two functions to add noise: add_noise has been renamed to add_noise_to_observations (similar to add_dipole_to_observations), and a new function add_noise now works on plain NumPy arrays
• Thanks to Numba, the 1/f noise generation is now ~10% faster and uses ~25% less memory
• The Simulation object now creates a new number generator (based on PCG64) for each MPI process
• New tests have been added for the noise generator (covering MPI as well)
• The functions add_white_noise and add_one_over_f_noise are now correctly exported

[ziotom78]

@paganol , apart from some speed/memory optimizations, this should fix #134. Does it look reasonable?

Unfortunately, to match the API used by other modules, I had to introduce a few breaking changes: add_noise is now add_noise_to_observations, and the parameter noisetype has been renamed noise_type.

[paganol]

Hi @ziotom78. It looks great to me. I made few corrections in the documentation. The only substantial change I made is that I put to zero the frequency zero in the 1/f model.

litebird_sim/litebird_sim/noise.py

Line 47 in 861d4c3

ft[0] = 0

If this is ok for you, we can merge it