Unfinished DipolarField functionality

[patrick-austin]

As noted in #83, the DipolarField class is unused in any of the commandline functions. Based on discussion with @leandro-liborio this was likely the start of an effort to replicate and extend MuSR functionality for dynamic effects, spinors, spin waves etc.

If this is a desirable, feature, this code should be fully analysed and development of this feature continued.