Move generate_bids_skeleton into lighter-weight repository

[tsalo]

What would you like to see added in this software?

The devs of BABS (@mattcieslak, @smeisler, etc.) are interested in creating a small package for testing BIDS Apps that would rely on generate_bids_skeleton. However, including niworkflows as a dependency just for this testing functionality has increased the size of the BABS container substantially (~1 GB to ~1.4 GB). If possible, it would be great to move the functions into something like nitest.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

We could definitely vendor the code and ensure that the licensing is handled correctly, if no one wants to split dataset-generation functionality out of niworkflows.

[effigies]

Niworkflows shouldn't be that large... Perhaps we just need to exclude test data from wheels?

I'm also okay with splitting into a separate package.

[mgxd]

Removing https://github.com/nipreps/niworkflows/blob/master/niworkflows/tests/data/testSpatialNormalizationRPTMovingWarpedImage.nii.gz might help...