Find all eigenvalues in a range

[AndreaIdini]

Once the States class has been implemented in issue #47 this is an easy enhancement.

@mechanapoleon this Find Energy routine can be easily modified to find all the eigenstates between e_min and e_max.
At the moment, it is a simple scan of the zero starting from e_min with mesh e_step. Once a change of sign has been found, it starts a bisection to find the correct Energy and break from loop.

Once the class States able to manage different states is introduced, it is easy to modify this routine to not break when the solution is found but to save the solution and continue scanning.

To be made issue

Originally posted by @AndreaIdini in https://github.com/Scienza/Schroedinger/diffs

[mechanapoleon]

Need to write another routine for finding the zero of the energy. Bisection can't fork for the harmonic oscillator.