Adding a new function: integralByInterval()

This will do the same as integral() funcion, except resetting the
total to 0 at the given time in the parameter "from”.
For example,
target=integralByInterval(company.sales.perMinute,”1d")&from=midnight-10
days
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.
Merge branch 'f_intergralbyinterval'

* f_intergralbyinterval:
  Adding a new function: integralByInterval()