dwifslpreproc: Don't discard modified dw_scheme
dwifslpreproc: Don't discard modified dw_scheme

Due to copying header properties from the original input DWI in order to preserve header key-values and appropriately concatenate "command_history", the contents of "dw_scheme" were being erroneously overwritten by the original gradient table of the input DWI. This fix ensures that the updated version generated during the script - whether including motion correction and/or explicit volume recombination - is propagated to the final output image.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3