Consider renaming ``StructuralReference`` interface

[oesteban]

Coming out from https://github.com/nipreps/niworkflows/pull/610#discussion_r570260072

Although, I'm still not sure StructuralReference is a bad name. We are indeed calculating "structural" images (as in, with the maximal structural contrast as possible).

If that argument is convincing, we could close this.

[effigies]

I think we should upstream the fix directly into RobustTemplate, though maybe triggered by a short_circuit boolean input, in case anybody depends on the failure condition. Note that this interface exists because mri_robust_template errors if only passed one image, not that it successfully does something we don't like or wastes time.

[oesteban]

Sounds good, a just one comment:

• Add the short_circuit boolean to the outputs too, for the case someone wants to implement different behaviors depending on whether there were one or more images at the input (asking for a friend).

BTW, we should be careful that these tweaks work well with MapNodes. I just discovered that this tweak - https://github.com/nipreps/niworkflows/blob/e2a86a43bd3b4330e9edd0257c422563d2b09f85/niworkflows/interfaces/fixes.py#L87-L133 doesn't work.

[effigies]

• Add the short_circuit boolean to the outputs too, for the case someone wants to implement different behaviors depending on whether there were one or more images at the input (asking for a friend).

So short_circuit on input is whether to enable short_circuiting and on output indicates whether the short circuit was used? e.g.,

outputs["short_circuit"] = self.inputs.short_circuit and len(self.inputs.in_files) == 1

I feel like I would rather not use the same name for two subtly different concepts, but as long as I understand what you want, we can quibble about naming when we go to implement.