Accept multiple EPIs/SBRefs for reference image calculation

[tsalo]

init_bold_reference_wf currently accepts a single SBRef image and a single EPI run. With multi-echo EPI, however, there will generally be one SBRef and one EPI run for each echo. The current idea for fMRIPrep is to use just the first echo's SBRef/EPI as the reference image, but I think that it's likely that at some point some method will be developed that leverages the information across echoes. Therefore, it might be good to make sure that init_bold_reference_wf can accept a list of EPI/SBRef images, even if it just selects the first image internally for now.

This references nipreps/fmriprep#1745 (comment).

[emdupre]

Just wanted to 👍 -- this approach makes sense to me !