fix(interfaces): Allow ``native`` grids (i.e. pass-through) for ``GenerateSamplingReference``
fix(interfaces): Allow ``native`` grids (i.e. pass-through) for ``GenerateSamplingReference``

This PR makes ``GenerateSamplingReference`` more flexible, accepting a
new input called ``resolution`` so that the decision about whether
generating a new resampling grid or not can be done in the context of
this interface.

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