Fix a precision problem

When comapring real numbers for equality one shouldn't compare the difference
of both values with zero. Due to rounding effects the result can be slightly
differnt from zero. If the difference of the values is smaller than a a small
value ( e.g. 0.00001 ) both values are equal.

sim/detectors/sts/CbmStsSimSensor.cxx

@@ -92,7 +92,7 @@ Int_t CbmStsSimSensor::ProcessPoint(const CbmStsPoint* point, Double_t eventTime
   Double_t tXav = 0.;
   Double_t tYav = 0.;
   //  Int_t    tZav = 0;
-  if (z2 - z1 != 0.) {
+  if (abs(z2 - z1) > 0.000001) {
     tXav = (x2 - x1) / (z2 - z1);
     tYav = (y2 - y1) / (z2 - z1);
     //  	tZav = 1;