Better support moving geometry

[inducer]

Some problems that currently crop up:

• [x] Recreating meshes is awkward. @alexfikl proposes Discretization.copy(mesh=...) (or .replace(mesh=...)). (which?) That seems easy and useful, but it requires remembering the group factory (probably OK).
• Meshes can't be created efficiently. Their data structure involves numpy at the moment. Maybe meshes should start using array contexts, too?
• [ ] Group factories need to recompute discretization node arrays. If the mesh already uses the right unit nodes, this can probably be avoided. (#135 )

cc @alexfikl