Encode affine Cartesian meshes for quad and hex `DGMulti` solvers

[jlchan]

Currently, we construct quad and hex meshes for DGMulti using a general VertexMappedMesh constructor. However, we assume constant geometric terms, which is only true of Cartesian (or parallelogram) quad/hex meshes.

This should be fixed by introducing a CartesianMesh type or an AffineMesh type parameter and restricting existing DGMulti Quad() and Hex() solvers to these meshes.

[ranocha]

restricting existing DGMulti Quad() and Hex() solvers to these meshes

This would remove the possibility to use some parallelogram meshes, wouldn't it? They are probably not that useful, but might be worth keeping. Thus, we could also specialize the implementation on CartesianMeshes instead.

[jlchan]

Sure - we could also introduce CartesianMesh as a constructor which creates something like an AffineMesh type, since the specialized implementations should pretty much be the same between them.

[jlchan]

Closed by #1019