Feature request: wrap function when wrapped=False in space.periodic and space.periodic_general

[cpgoodri]

When passing wrapped=False to the periodic space functions, it seems that in addition to displacement and shift, it is important to define a third function that allows the user to put the particles back in the box. A typical use case would be to use unwrapped positions during a simulation because they are (presumably) faster, and then do something like R = wrap(state.position) to get the final positions. My thinking is that there are two options for the api:

option 1:
displacement, shift = space.periodic(L)
displacement, shift, wrap = space.periodic(L, wrapped=False)

option 2:
displacement, shift = space.periodic(L)
displacement, shift = space.periodic(L, wrapped=False)
displacement, shift, wrap = space.periodic(L, wrapped=False, wrap_function=True)

The second option might be preferable since it would be backwards compatible. I would be happy to take a first shot at this if that would be helpful.