Path to a revamped tutorial

[mlincett]

We have received multiple feedbacks about the current tutorial being somehow hard to undersand.

In addition it has some minor mistakes (k_to_flux and flux_to_k are swapped), it currently fails to execute (see #286) and defaults to standard likelihood instead of standard_matrix without any discussion on what is appropriate where (see #14).

I have since try to address these points and I am ready to update the tutorial.

There are however a several situations where we need to over-explain things just because the way things are in flarestack is counter intuitive or still far from optimal:

• the name FLARESTACK_SCRATCH_DIR seems to imply that the path will be exclusively used by flarestack, but then everything is put under a flarestack__data subdirectory meaning FLARESTACK_SCRATCH_DIR should rather be a generic/shared scratch space;
• there can be confusion between flarestack__data and FLARESTACK_DATASET_DIR;
• importing a flarestack module will trigger the creation of a flarestack__data directory structure, I'd like my imports to be read-only operations;
• there is some ambiguity in relation to what is a name, sometimes name implies a path (such in the case of analyses, catalogues...)
• flarestack.shared.plot_output_dir is a function rather than a variable, so it should better named get_plot_output_dir();
• flarestack.utils.prepare_catalogue.ps_catalogue_name has a rather obscure naming, something like create_catalogue would be more understandable.