Multiple discretization handling

[mloubout]

We currently have a fairly ad-hic handling of multiple "grids" (Subdiomains, Grid, ...) in the operator argument processing

A proper Discretization hierarchy needs to be implemented so we can easily check if a discretization is a subpart of another one. See here for details

https://github.com/devitocodes/devito/pull/2205#discussion_r1331583608

This is linked to the functions on subdomain current work