Fmriprep: No default output spaces in 20.0.0rc1

[effigies]

Up to now we've always had MNI152NLin2009cAsym as a default output space. Now I get

UserWarning: fMRIPrep will not generate preprocessed derivatives because none of `--output-spaces`, `--use-aroma`, or `--cifti-output` were set.

I would suggest we restore the default behavior.

[mgxd]

This is the intended behavior, as the large refactor of output spaces makes it possible to add MNI152NLin2009cAsym normalization for the carpetplot generation and keep the workflow from breaking, but not clutter the output directory with (possibly) unwanted files.

And I believe it will still generate preprocessed anatomical? just not any func/ outputs.

[effigies]

The carpetplot bit was written for MNI152etc because that was the default output space, not the other way around. I'm happy to let users override the default space but having no BOLD outputs by default seems pretty extreme.

[oesteban]

There needs to be a mechanism to allow users not to generate any outputs (although I don't really understand why you would do that).

One thing we could do is:

• Roll back to the original behavior (i.e., if no --output-spaces is given, output MNI152NLin2009cAsym results).
• Allow the user to write --output-spaces without any value, to indicate no bold conversions are to be generated.

WDYT?

[effigies]

That sounds fine.