fix(interface): fields were being replaced in outputs call
fix(interface): fields were being replaced in outputs call

A copy of the fields list was necessary for the default behaviour to work (i.e., there's only one input image called ``in_file`` and the corresponding output is ``out_file``).

poldracklab/niworkflows,oesteban/niworkflows,poldracklab/niworkflows,oesteban/niworkflows,oesteban/niworkflows