Do not interpolate if there were missing data points for a long time.
Added interpolation window to limit the interval for interplation.
- Interpolates a data point within the interpolation window of two adjacent
  data points.
- No interpolated data points if the time gap of data points is bigger than
  the interpolation window.
- Discards rates if their time intervals are bigger than the interpolation window.
- "iw-" is the query param for the interpolation window.
- Added interpolation window web interface.
- Supported zero interpolation window to turn off interpolation and
  rate computation consequently.