ENH: Added normalisation condition for finite well.

FiniteWellBasis.eigfunc now returns a normalised eigenfunction.
ENH: Finite well -- set V(infinity) = 0.

Before, the bottom of the well was at 0 and the top at V0. While this is
what seems to be used on Wikipedia, I think it's best to have the bottom
at -V0 and the top at 0.
DOC: Added examples for finite well.
TST: Further tests for finite well.

* Basic eigenfunction value testing.
* Eigenfunction normalisation.