Prevent Node unbiasedResidual returning a negative number

Sometimes, one would see an FPE in OTTrackMonitor due to an Outlier Node returning a negative number in this line:

https://gitlab.cern.ch/lhcb/Rec/blob/master/Tr/TrackMonitors/src/OTTrackMonitor.cpp#L482

This can be avoided making the errUnbiasedResidual always return a positive number.