Wrap DMFieldEvaluate and expose it to users

[julesghub]

DMFieldEvaluate looks like it works with DMPlex and retrieves values, 1st and 2nd derivatives for a field. https://petsc.org/release/manualpages/DM/DMFieldEvaluate/

Yes to having access to this feature for visualisation, model checking and debugging. Wrapping it involves some extra cython code and a way to pythonically caputure the output data.

@bknight1