ENH: Start of finite well implementation.

Basic functions to find the eigenvalues.
ENH: Mostly working implementation of 1d finite well.

The eigenfunctions are not normalised, but apart from that, they are
correct.
ENH: minor improvements to finwell.

* raises ValueError when asked for an unound eigenfunction.
* added test for eigenvalues.
* made attributes _eigvals and _*_brackets explicitly private.
DOC: Added docstring for FiniteWellBasis class.

And a few implementation details.
DOC: Added finite-well stub to documentation.
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.
Merge pull request #1 from scikit-quantum/finite-well

Basis class for a 1D finite-well potential.