Add `with_order` to `FunctionSpace`

[inducer]

Inspired by this comment https://github.com/inducer/meshmode/pull/291/files#r777549852 from @alexfikl

If when/done: use it in the spots in the PR that provided the inspiration.

[inducer]

Problem: It probably shouldn't be in FunctionSpace, since order isn't necessarily a well-defined concept for every function space (e.g. anisotropic-order tensor product).

[alexfikl]

Hm, yeah, forgot about that. Maybe something narrower then? At least for the use cases in meshmode

• mp.space_for_shape(shape, 1): just needs something to interpolate to vertices.
• mp.space_for_shape(shape, 2) in tesselation: seems to mostly be used as a way to bisect the element. This will probably fail if we try to use PN on a square anyway?
• mp.space_for_shape(shape, order) in visualization: VTK only supports isotropic things anyway, at least for now.

Not sure, probably better to wait until things start breaking 😁