Added invalid beam skip

When the current particle set is converged the pf sensor update will skip laser scans that don't agree with at least a given threshold of samples. 

Enable with do_beamskip=true
with beam_skip_distance being the distance threshold (above which a scan is considered invalid)
and beam_skip_threshold being the ratio below which a scan is considered invalid (for < beam_skip_threshold number of particles - the scan didn't agree with the map).
Move to line 72 above to have all hard-coded numbers in one place.