Use of periodic boundary condition in geometries

[henriasv]

It is quite simple to implement a periodic boundary condition for the sphere geometry. Actually, for any geometry that is based on the minimum image convention vector between control point and particle positions, a periodic bounady condition geometry is well defined. It is also simple to implement, since ase has a method tha lets us calculate the distance between pints and particles.

For geometries involving planes and lines however, it is not that obvious, i think, how to implement a periodic boundary condition. For now, I think we should move the periodic boundary condition logic down to the specific classes where this is an option (currently only sphere, but will also be ellipsoid.)