Add commons potentials

[Luthaf]

This is a meta-issue about potentials, the code would need a lot of them :

Short range

• [ ] Morse potential

Biochem potentials

• [ ] CHARMM
• [ ] AMBER

Bonds

• [x] harmonic

Angles

• [x] harmonic
• [x] cosine - harmonic

Dihedral angles

• [x] Torsion
• [ ] Cosine series

The potential interface should also change to take a vector between two, three or four points and return a vector of the force or the energy.

[Luthaf]

In relation with #2, the potential code is going to be separated in two :

• potential functions (the form of the functions)
  • Lennard-Jones
  • Harmonic
  • ...
• potential compuations (the way to compute the values)
  • directly
  • shifted with cutoff
  • using tables
  • using Ewald's sum

[Luthaf]

This was done in 9ee0a2f8c9669a26a99ed9100b15979c774d5982