Add first differences to dataset.

[EdGarrity]

y(t) = [x(t) - x(t-1)] / x(t - 1)

Suggested implementation:

Modify Broker::get_value_from_datatable() to return first differences instead of first derivative if column < datatablecolumns . Should also change the type of the column parameter from int to long int to be sure the parameter can hold lager numbers. Also update the type of the PushP integer stack from int to long int.

[EdGarrity]

See https://stackoverflow.com/questions/1819189/what-range-of-values-can-integer-types-store-in-c