Output spaces, output references, FreeSurfer spaces

This conversation was initiated in poldracklab/fmriprep#1334

This issue is just to point out that a uniform interface to outputs is necessary, as regards to templates.

At the very least, the ``fs_templates`` variable in sMRIPrep should be revised (rolled back to `output_spaces` if we decide not to change this, or add a corresponding `volume_spaces` with all the handling logic).