We should clear spatialindex when moving the mesh

[stephankramer]

Consider the following code:

from firedrake import *

mesh = UnitSquareMesh(10, 10)
V = FunctionSpace(mesh, "CG", 1)
x, y = SpatialCoordinate(mesh)
u = Function(V)
u.interpolate(x)

print(u.at([0.5, 0.5]))
mesh.coordinates.dat.data[:] **= 2
print(u.at((0.5, 0.5)))

The second point interpolation at (0.5, 0.5) fails with a PointNotInDomainError. This is because it uses a spatialindex that got created and cached on the first point interpolation call and thus returns candidate elements in which the point would have originally been located, but these elements have since moved elsewhere. Firedrake then tries these elements, and finds none of them contain the queried point. This can be fixed by call mesh.clear_spatialindex() after each move.

Note that this specifically only addresses interpolating in individual points via the at() (or __call__()) method. I believe function-to-function interpolate now uses VertexOnlyMesh. I don't currently have any evidence that is not doing the right thing, but we should be really careful as AFAIK this stuff isn't really supported (if the warning here https://www.firedrakeproject.org/mesh-coordinates.html is still valid) but we should add some tests to verify it is working as we expect, and to ensure it isn't broken in the future. The same goes for when we use cross-mesh project() as this probably involves a spatial index that is stored in yet another place (super mesh).