dsl: Introduce ability to define Functions on Subdomains

[EdCaunt]

Add Functions on SubDomains functionality.

Remaining todo:

• Tutorial notebook showing
  • Demo for seafloor-like configuration
  • Demo for damping-like configuration
  • Demo for snapshotting only one region of the grid
• Change subdomain numbering to be more robust and prevent memory leaks
• Tests for propagator on subdomain
  • Including with MPI