Use 2D bins in z-k

[andreufont]

The code currently assumes that we have Nz independent bins in redshift, each of them with the same Nk bins in k.

However, the k-binning could be different in each z-bin (see #14) and the z-bins are not always independent (see #12).

We could tackle both issues together by allowing a data vector that includes all k-bins of all z-bins, with a large covariance matrix that can correlate all of them.

[jchavesmontero]

It has already been addressed