Replace .at() interpolation in 2D callbacks

[cpjordan]

.at() raises an error when the number of processors used is large, use a VertexOnlyMesh instead for interpolation

[cpjordan]

Prevents the Firedrake issue #2790 whilst Firedrake issue #3080 is waiting to be resolved. This method is slower than .at() so an alternative is to add a try except with .at() before moving onto using a VertexOnlyMesh.

There are some remaining direct applications of .at() but they are for 3D examples where either the callbacks are only being used in serial or where the problem is so large that .at() is functioning fine.

[tkarna]

Thanks, looks good. Only minor comments

[cpjordan]

The current version of Firedrake is failing, which seems to be causing all the Thetis tests to fail. Waiting for this to be updated.

[tkarna]

Thanks!