Missing bits of the gravity code

Summary of my whiteboard.

Items missing for full code:

• Wrapped multipoles for CIC (#353 (closed)) (done in master !433 (merged)),
• Correct multipole drift (done in master !433 (merged)),
• cell_is_active() separately for gravity and hydro (done in master !433 (merged)),
• MPI pairs (done in master !433 (merged)),
• Sigmoid in M2L (implemented in !566 (merged)),
• Sigmoid in M2P (implemented in !566 (merged)),
• Compute the potential for gparts (implemented in !512 (merged)),
• Cosmology (half implemented in branch cosmology. (done in master !509 (merged)),
• Simplify 1st order multipoles (implemented in !604 (merged)),
• Symmetric M2L (implemented in !604 (merged)).

For later:

• Good gravity rebuild criterion.