Modify regrid function to allow regridding in many steps

[juliettelavoie]

Generic Issue

I want to modify regrid to be able to do a regridding in a few steps using intermediary grids.

Eg. CMIP6 -> 1deg regular grid -> 0.5deg regular grid -> era5-land

As CMIP6 -> era5-land is a huge step, it is better to do it in a few steps to avoid weird features. Right now, I am doing this in the workflow, but I want it to be inside the xscen function.