Calculate the required parameters for intensity analysis
reqpar(time.points)a charachter vector showing the time point of each raster layer in chronological order.
The output is a list inclusing: a one element vector showing the number of intervals, a charachter vector showing the initial times for all intervals, a charachter vector showing the subsequent times for all intervals, an integer vector showing the duration of each interval and a charachter vector showing the label of each interval.
reqpar function gets the time point and return a list containing required variables used in intensity analysis.