added code that computes the headway when a car begins entering the lane for a lane change. Also added code that divides each bin in the histogram by the total number of counts in that bin for the whole span of NGSIM data, which normalizes the counts by their likelihood of appearing
added code that computes the headway when a car begins entering the lane for a lane change. Also added code that divides each bin in the histogram by the total number of counts in that bin for the whole span of NGSIM data, which normalizes the counts by their likelihood of appearing

cathywu/flow,cathywu/flow