Add quartic single ion anisotropy

[mducle]

Currently SpinW supports a quadratic single-ion anisotropy term:

This allows to define axial and planar anisotropy.

A quartic single-ion anisotropy term, like:

analogous to the biquadratic interaction:

would allow more complex anisotropy (such as fan or conical types) which could manifest in rare earth systems.

The implementation of the current (quadratic) anisotropy is to add the SIA term to the diagonal of the JJ interaction matrix. A similar approach might be possible for a quartic anisotropy (by adding to the bq biquadratic interaction matrix).