The definitive metadata of a MeshCoord is just mesh, location + axis.
We have blocked any assignment to these, since it makes no good sense to change any of them.

However, in generic code this can be a problem since no-ops to reassign metadata will now also fail.
As observed in this line while implementing #4651 : even if we ensure that this will be a degenerate "no-op" for meshcoords, it still fails.

Possibly, we should allow assignment, but only with no-change values.