Add direct=False gap-filling
Gap filing forward method has been changed. The argument direct=false enables to re-interpolate a time series that has already been interpolated on the target_doy to the acquisition dates. To do so, the following main modifications have been made:
- linear_gap_filling function now requires target_doy to be [B,N,...]
- due to the modifications of the linear_gap_filling_method, the tests have been updated
- in the direct=false. The time series is interpolated along the acquisition date given by sits.doy. Therefore, the class SITS does not have the requirement that temporal dimension of the sits and doy should match. I am not really satisfied by that.
- A test of the forward method with direct=false have been implemented