Would be nice for bodies to shed visual particles to illustrate wakes

[markstock]

If every boundary element had a small chance every time step of generating a new inert particle. we could visualize wakes more interestingly (to many people) than with vorticity. It would recreate the look of releasing milk from an object in a water tunnel.

[markstock]

This may be easier to support once we open up the possibility of any of [Boundary|Flow|Measure] feature generating any of [Points|Surfaces|Volumes] computational elements. Because right now, Boundary features only generate Surfaces, and Flow and Measure features only generate Points. When all options are supported, an "emitting" measure feature could be defined on a Surface, and that Surface could be an already-existing one.