adding an integralByInterval() function

This will do the same as integral() function, except resetting the total to 0 periodicaly.

For example:

     target=integralByInterval(company.sales.perMinute,"1d")&from=midnight-10days

This would start at zero on the left side of the graph, adding the sales each minute, and show the evolution of sales per day during the last 10 days.