[processing] fixes SAGA raster import/export

The parameter "-TRANSFORM" that was added in 1d754d9 (because now is mandatory in SAGA 2.10) was added in the command that exports the SAGA grid into a TIFF, but actually had to be added in the SAGA command that imports a tiff into a SAGA grid. It was anyway missing a space and the process was failing (with the missing space it would have failed too, because the parameter would not have been recognized). In the commit I also propose to use the "-INTERPOL 0" parameter, to import the rasters into SAGA grids using a NN interpolation instead of the B-Spline used by default by SAGA 2.10.

SAGA raster export was also modified in order to allow the "RGB composite" module create the right output.