KIM computations work again.

And now they even support PBCs in all neighbor modes.
Efficient neighbor list for MI_OPBC_F.

No more ghost atoms for that mode.  Disadvantage is that the box size
must exceed 2*cutoff.
Method to update rvec array without complete neighbor list recalculation.
Don't store ghost atoms for NEIGH_RVEC_F.

They are not needed because of the rvec array.