some fixes to velocity advection  to somewhat solve offset shifts  still could use some work
added searchRadius  attr to feed  kdtree lookups  to fix  low particle count searches where they  overstep their local surroundings
added veloctiyStretch  to spread  expanded particles down velo axis
velocity advection now seems basically finished  for now.
on to hole filling and then arnold rendering/translation support
added  input support for FPS for velocity stepping