add price volume trend indicator

[sp98]

Reference: https://www.investopedia.com/terms/v/vptindicator.asp

Formula Used:

1. For second candle:
   [((CurrentClose - PreviousClose) / PreviousClose) x Volume] + 0 (because there is no PVT to first candle)

2. For subsequent candles:
   [((CurrentClose - PreviousClose) / PreviousClose) x Volume] + PreviousPVT

Usage:

	closePrices := techan.NewClosePriceIndicator(series)
	volume := techan.NewVolumeIndicator(series)

	pvtIndicator := techan.NewPriceVolumeTrendIndicator(closePrices, volume, 1)
	fmt.Println("PVT - ", pvtIndicator.Calculate(n)

	pvtSignalIndicator := techan.NewPVTAndSignalIndicator(pvtIndicator, techan.NewEMAIndicator(pvtIndicator, 21))
	fmt.Println("PVT - Signal - ", pvtSignalIndicator.Calculate(n)

Where n is the index. It should be size of the series.

Signed-off-by: Santosh Pillai sapillai@redhat.com

[sdcoffey]

Thanks @sp98! Will take a look at this ASAP