Introduce a managing of states

[AndreaIdini]

At the moment, the program gets only one state composed of

• energy
• wavefunction

It is needed to introduce a system to find a mange all the states in a given range.

[mechanapoleon]

In order to solve the issue, it may be a good thing to introduce the following:

• A class containing some standard boundary conditions (zero, periodic,...);
• Some perturbative expansion to study the energy spectrum around the minima for the harder cases.