Solver Class

[AndreaIdini]

Implement the solver class:

• interface method: keep the option open to use more solvers
• modify the structure to be more "C++ like" (less pointers and list, more std::)
• prototype how to handle the centrifugal term for spherical and cilindrical, 3D and 2D

[AndreaIdini]

From @Pandry in #43

I don't believe that printToFile should belong to this class (Numerov ndR), in the future I believe it should be used something like an interface to do so...
At least a TODO could be written

[GabrielePisciotta]

• Solver class (Numerov) is now C++-like
• No more printing (the class State is the new responsible for that)

[AndreaIdini]

how to handle the centrifugal term for spherical and cilindrical, 3D and 2D

The spherical term is L^2/2mr^2 (cf. http://www.fuw.edu.pl/~dobaczew/TALENT_DFT_2016/main.pdf) but in general one should transform the basis in 2D and 3D cases, and consider cylindrical symmetry also in 3D.