Smarter test of multiecho datasets

[oesteban]

Instead of:

https://github.com/poldracklab/fmriprep/blob/4c54ca4f855d4f560c7c4abdbbbcb4fb3a60bae6/fmriprep/workflows/bold/base.py#L147

we may want to do something like:

entities = layout.parse_file_entities(ensure_list(ref_file)[0])
multiecho = "echo" in entities

May I suggest to address this in the context of #1803 ?

[tsalo]

Sorry for the delay on responding to this. Is echo prohibited for single-echo data? The specification is unclear:

Multi-echo data MUST be split into one file per echo. Each file shares the same name with the exception of the _echo-<index> key/value.

If we assume that single-echo data cannot include echo, then I think the proposed check is good.

[oesteban]

Perhaps this is easier than I thought in the beginning. What about testing:

multiecho = len(entities.pop("echo", [])) > 2

[oesteban]

I believe that covers one of the edge cases of #2182 too.