Merged gravity M-M tasks

Merge the M-M tasks between two cells into one single M-M task that will perform all the operations on the pair's progenitors.

Additional changes are:

• Delay the drifting of the multipole to the task instead of the unskip.
• Use the CoM at rebuild time in cell_can_use_pair_mm_rebuild() as the name indicates.
• Add extra timers to engine_maketasks()
• Use the list of local cells (instead of all cells) in space_regrid() to infer the maximal smoothing length.
• Fix the plotting scripts for matplotlib 2.x.
• Change the task flag to a long long instead of int.