Calculates the price of previous year for chained price quarterly series
xpxbase=ann_overlap_2_baseprice(xvolch,xval)
* xvolch = a quarterly series in volume at the chained prices of the previous year
* xval = a series in values
* xpxbase = a quarterly ts, the corresponding price of previous year (filled with ones when the series are %nan or the volume series is 0)