Modified the density convergence code to force bisection to occur after BISECTIONITER (defined in allvars.h) iterations.  This should guarantee convergence (even if it's a bit slower, hence doing a few loops the normal way first) and prevent crashes from convergence issues in 2D in particular.  Also fixed up the CUTOFF_RADIUS compilation option, which must be used in conjuction with SINK_PARTICLES and will delete any particle that is more than CutoffRadius (parameter) from the center of mass of the system.