Maybe switch to mappings that accept vectors/tuples for CurvedMesh/P4estMesh

[efaulhaber]

    node_coordinates[:, i, j, tree] .= mapping(node_coordinates[1, i, j, tree],
                                               node_coordinates[2, i, j, tree])

I wonder whether we should switch to mappings that accept the coordinates as a single SVector/Tuple/AbstractVector/iterable... Then, we could just use identity for the identity mapping... However, this won't block this PR and we might continue this discussion elsewhere.

Originally posted by @ranocha in #598 (comment)

[ranocha]

Triage: Mappings should accept the coordinates bundled in a tuple/vector

[ranocha]

After a closer look at the source code, I don't think this change is really necessary. We would need to wrap and unwrap everything without gaining much. In addition, it's easier to distinguish 2D and 3D mappings using the current signature. Thus, let's not make this breaking change now.