class_docstr: Force Index (FI)
  
  It illustrates how strong the actual buying or selling pressure is. High
  positive values mean there is a strong rising trend, and low values signify
  a strong downward trend.
  
  http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:force_index
  
  Args:
      close(pandas.Series): dataset 'Close' column.
      volume(pandas.Series): dataset 'Volume' column.
      window(int): n period.
      fillna(bool): if True, fill nan values.
code: def _run(self):
      fi_series = (self._close - self._close.shift(1)) * self._volume
      self._fi = _ema(fi_series, self._window, fillna=self._fillna)
  
imports: import typing
  import numpy
  import pandas
  from ta.utils import IndicatorMixin, _ema
label_desc: Use image node_id 2 for calling the ForceIndexIndicator obj's underlying member method code with example usage: obj._run() without return types