This broke recently (or never worked), because nonconforming
discretizations were not tested anywhere.  This patch therefore
also generalizes harmonicmaptest to also test with a
nonconforming discretization.

As part of the fix, the interpolation rules get a new method
`evaluateValueAndDerivative`, because the previous way to get
the value and the derivative in a single call (the `evaluateDerivative`
method that takes a value as an argument) only worked for the
conforming case.