add unit/integrted tests for dpc

The images do not have to be random, just deterministic. 

Presumably you can generate some sensible data for the reference image, then generate a phase field, forward propagate the reference through that and then make sure you get the right answer back back out of the DPC code.

Comparing the output against the C-code is useful (and must be done!) but it is not the sort of tests that we need in the test suite.