Check native_quantity filter and area

[combet]

• Check if it is possible to filter directly on the native quanitites for SkySim and if so update the base notebooks to do that.
• Also, check how to get the survey area from the catalog object (At the moment, using hard coded 50 deg2 as reported on the catalog description in Confluence, but it's probably rounded)
• Update the notebooks accordingly

[combet]

• There is indeed a sky_area information attached to the catalog. The notebooks have been updated to use that
• While there is a native_filter option to get_quantities from GCRCatalog, trying to use it to filter on the baseDC2/sod_halo_mass (i.e. M200c) field fails with: Not all quantities in native_filters can be used in native filters!. So unfortunately, we'll keep first loading the catalog using a lose filter on Mfof, that can be later refined on the M200c mass.