Add traits for energy computation

[Luthaf]

The following traits should be added:

trait Energy {
    // Used in monte-carlo and energy computation
    fn energy(&mut self, system: &System) -> f64;
}

trait Forces {
    // Used in virial computation
    fn forces(&mut self, system: &System) -> Vec<Vector3D>;
}

trait Energetics: Energy + Forces {
    // Used in energy minimization
    fn forces_and_energy(&mut self, system: &System) -> (f64, Vec<Vector3D>) {
        (self.energy(system), self.forces(system))
    }
}

This would allow to:

• Have a SimpleEnergetics mode where we do not re-compute periodic boundary conditions when needing both forces and energy;
• Add other way of computing energy, like ThreadedEnergetics, VerletLists or GPUEnergetics