Suggestion: Negative Volume Index can be computed without for loop

[cristiangreco94]

Suggested implementation

class NegativeVolumeIndexIndicator(IndicatorMixin):

def __init__(self, close: pd.Series, volume: pd.Series, fillna: bool = False):
    self._close = close
    self._volume = volume
    self._fillna = fillna
    self._run()

def _run(self):
    price_change = self._close.pct_change()
    vol_decrease = self._volume.shift(1) > self._volume
    nvi = np.where( 
        vol_decrease, 1+price_change , 1
     )
    nvi[0] = 1000.
    self._nvi = pd.Series(
        data=nvi.cumprod(), index=self._close.index, dtype="float64", name="nvi"
    )