Script for covariance from Gaussian simulations

[sachaguer]

This PR implements a script from the previous non-tomographic implementation (see issue #282).
The script relies on the cosmo_val cat_config such that it can be run in parallel of cosmo_val.
The Cl evaluations are parallelised with MPI to efficiently get samples.

The pipeline is now running without Runtime errors but remains to be validated against the iNKA implementation.

I will add a plot when the latter validation is ready and ping for reviews.

[sachaguer]

I fixed a few bugs in the implementation that now returns sensible outputs for the pseudo-Cl samples from the Gaussian simulations (see e.g. the samples against the fiducial for noise-bias subtracted auto-bins below).

When the samples will be finished, a final runtime validation of the merged covariance will be possible and a comparison with iNKA.

[sachaguer]

Progress report: the first covariance from Gaussian simulations has been produced:

Still there is a big disagreement between the simulations and iNKA that I need to pin down

[sachaguer]

Progress report: iNKA and Gaussian sims agree on the non-tomographic case. Tomographic case is running. There is no reason why they should disagree so this should be nice final validation showing convergence of the code.

[sachaguer]

This circles nicely from the bug reported in #291 to the validation of the Gaussian simulated covariance and the GLASS mocks.

The bug fixed on the noise bias propagation for the iNKA covariance looks much better (see below)

I shared the non-tomographic agreement between the Gaussian sims and the iNKA covariance above. This agreement remains in the tomographic case (see figure below for the bin 6-6 of the test mock used, I also checked cross-bins and other auto-correlations)

I think this finalises the validation of #190 #286 #291. @cailmdaley this PR can be reviewed.