Exemplar dataset and derivatives #59

I haven't made any progress on processing and sharing derivatives for the open multi-echo datasets, but I was thinking that an easier lift that might be useful for testing and for the multi-echo-data-analysis Jupyter Book.

I see this as resulting in three OpenNeuro datasets:

1. The raw OpenNeuro dataset, with one subject (and maybe limited to one session?) from each open multi-echo dataset.
   • This will require some curation, to deal with any inheritance issues.
2. fMRIPrep derivatives
3. AFNI derivatives