Fix missing clusters split while jumping back

[HaoQChen]

ScanProcessor::splitConnected(float thresh) currently only consider the situation that range increase suddenly but ignore sudden decrease. When sudden decrease occurs, it will jump to "else" to "++s_rest". If range is far enough, "expand" will be 1 or 2, nothing happen. But if range is very close, it may miss a jumping back， which will led to incorrect clusters split.

I think we just need to add a "std::abs(float)" to the "if"

[HaoQChen]

leg_detector->void pairLegs(). At the second loop that "Attempt to pair up legs with no id", did you forget to change "closest_dist" to "d" when a pair of better legs appear. If we don't do that, it will only remember the first pair and every thing else we do will be forgot. And I think we just need to loop "leg2" from "leg1 + 1", cause the distance is scalar not vector.