LHCBGAUSS-876: Fixed Bug in RadLengthColl in GaussTools

Fixed the issue where cumulative radiation length and interaction length counters were not being reset when the end of the geometry was reached. Fix involves adding an additional variable which holds the previous step Z position, this is then compared to the current step Z value; if the Z_new < Z_prev the counters are reset. Has been tested using the lhcb-sim09 nightly slot.