Compute molecule center of mass for periodic systems

[avmarchenko]

Currently not implemented (if periodic = True in Universe.frame, Universe.compute_molecule_com will raise an error). A workaround (if molecules are not spliced in the periodic boundary condition is to set periodic to False.

To fix this issue, need to account for spliced molecules in periodic boundary conditions - some of this machinery has been implemented as part of the "visual atom" table. Secondly, may/may not need to rework the "projected atom" table (the 3x3x3 projection of the periodic unit atom table).