DOC: Added docstring for Busch model constructor.
DOC: Added reference to Busch model.
ENH: Added normalization for Busch eigfuncs.

The normalisation is done numerically. Mathematica refuses to give me a
sensible result...
DOC: Fixed Busch docstring following normalisation.
DOC: Documented members of BuschBasis class.
TST: Added some tests for Busch wavefunctions.