Refactored directory structure for Python wrapper

See merge request !3

Added flags for the various terms in the covariance

See merge request !2

implemented covariance for 2 populations

See merge request !1

Now the user can choose between A_s and sigma8. Note that the Python interface
doesn't change A_s if sigma8 changes, and vice versa; this is due to the
complexity of implementing such a routine, which would also require the use of
scipy instead of just numpy.

When setting `m_ncdm`, the value of `Omega0_nu` wasn't being set accordingly.

More specifically:
- T_cmb
- m_ncdm
- N_ur
- N_ncdm

In line with the principle of least surprise, all literal integers should have
explicit dots when part of floating math.

With just density, the l=2,4 multipoles are extremely small, and the GSL
integrator would abort the computation, so now I just turn it off for both
`compute_corrfunc` and `compute_multipole`.

For now, it only has the `covariance_matrix` function, but I'm sure more will
be added.

It's useless on its own anyway, and may confuse users if they accidentally
instantiate it.

It requires the entirety of scipy as a dependency, so it's best to put it in a
separate module.

According to PEP 518, one needs to declare Cython in this file, instead of in
setup.py.

Hopefully everything is being changed consistently now.

Mostly modified some explicit casting, and it all works.

Now the signature of the functions is available when using Coffe interactively,
or when generating the docs.

This is to be able to add the `covariance_matrix` method, which returns the
block-diagonal matrix that can be used for, say, Fisher forecasts.