solve_radial_eigenproblem: expose the P, Q funcs

The normalized P, Q wavefunctions are now exposed directly. The docstring was updated as well.
This way the user can decide what to do with them (e.g. plot them individually,
or construct a charge density).
Update the rest of the code with the new interface

Now the whole code base compiles and works with the new interface of
solve_radial_eigenproblem().
Update the C interface
Update the Python interface
Merge pull request #5 from certik/exposePQ

Expose P and Q from solve_radial_eigenproblem()