Calculation of average volume at current time for a number of previous sessions, known as Average Volume at Time (AVAT).
Inputs:
* period to use for accumulation. "D" is the default value, useful to view data for each session.
* number of previous sessions to average
TODO: more intelligent accumulation of number of bars in a session, since there may be sessions with different values
TODO: interpolate volume according to current time, inside of the last bar