dwi2tensor: Preserve dw_scheme for -predicted_signal

Preserving the diffusion gradient scheme in the image header for the output of the -predicted_signal option is preferable for enabling visualisation of such data using the MRView ODF Tool in Dixel mode.

cmd/dwi2tensor.cpp

@@ -220,14 +220,14 @@ void run ()
   Header header (dwi);
   header.datatype() = DataType::Float32;
   header.ndim() = 4;
-  DWI::stash_DW_scheme (header, grad);
   PhaseEncoding::clear_scheme (header);
 
   Image<value_type> predict;
   opt = get_options ("predicted_signal");
   if (opt.size())
     predict = Image<value_type>::create (opt[0][0], header);
 
+  DWI::stash_DW_scheme (header, grad);
   header.size(3) = 6;
   auto dt = Image<value_type>::create (argument[1], header);