Fix parent of imported category (#62)

* Change parent of ATOM_SITE_FOURIER_WAVE_VECTOR to MAGNETIC.

Since the CIF_MS dictionary is imported in full (head-in-head import),
all top level categories from the CIF_MS dictionary become automatically
reparented from MS_GROUP to MAGNETIC. Furthermore, the MS_GROUP head category
is not imported at all, thus resulting in a missing parent category.

* Reorder categories in accordance to the style guide.