Spike in COM pulling force at checkpoint continuation with geometry=direction-periodic
Fix pull bug with geometry=direction-periodic
When doing COM pulling with geometry direction-periodic,
the reference location needs to be used to compute the current
distance. This reference was taken from the previous step instead
of the current. In practice this only caused issues at restarts
when the distance was more than half the box length.
Organized the 19 members of pull_coord_work_t into 5 sub-groups
and moved some into a new external PullCoordVectorForces struct.
Added a constructor and changed the pointer to coord in pull_t
Also fixes #2446