Added option to fix bug that resets particle weights on motion model updates

There's a bug in amcl's motion model--the particle weights are reset in the motion model after the particles are propagated.  This isn't a problem if resampling happens on every step, but if you are using a resample_interval >1, amcl will essentially ignore all but the last observation before the resample.  

Just in case anyone is using resample_interval>1 and has a system working *with* the bug, we made it an optional param that defaults to the old behavior.  (Feel free to debate which way it should go, or whether it should just be a fix and not a param at all.)