Refactor `mesh.py`

[leo-collins]

I think we should refactor mesh.py. It is currently over 5000 lines long, and contains normal mesh stuff, mesh IO stuff, submesh stuff, vertex only mesh stuff, extruded mesh stuff, etc.

I propose we create a new sub package mesh, which contains several modules, something like

firedrake/
  mesh/
    __init__.py
    base.py
    extruded.py
    submesh.py
    vertexonly.py

I think this is far more logical and maintainable that what we currently have.

[connorjward]

1000% yes this should happen

DO NOT DO THIS BEFORE PYOP3

[connorjward]

Other possible bits to split out are topology vs geometry, IO, utility meshes