"Particle escape" handling for DMSwarm

[julesghub]

This hasn't been tried or tested.
What happens when a particle advects beyonds the mesh, or beyond the shadow space.

[lmoresi]

Tricky one !

Can we create an actual issue here. We know there are problems in dmplex when trying to find particles without good hints about process / element but we need to give examples that the PETSc team can raise as issues themselves.

[knepley]

The original design did not support particles hopping across nearest neighbors. Is that what you are seeing? We are fixing this now. Also, there was a bug in quadrilateral location that I just fixed.

[lmoresi]

Exactly what we are seeing and we have a kd-tree hack that we are currently using. We'd really like to have this disappear into dmplex and not our bespoke (crappy) code.

[julesghub]

Sounds great! Keep us posted with this @knepley