Support for interactions in Fourier-space

[kbarros]

Sunny should accept specification of spin-spin interactions with a Fourier space function $J(\mathbf{q})$. This would encompass both RKKY or long-range dipole-dipole interactions.

In the other direction: Given real-space interactions that are bilinear in spin, Sunny could extract the corresponding $J(\mathbf{q})$ function. This could be useful for purposes of visualization or stability analysis.

[kbarros]

#257 adds support for dipole-dipole, and opens the door to specification of arbitrary $J(\mathbf{q})$.