handle multi input in case of func-task, func-regressors, and fmap

[giulia-berto]

So far, bl2bids can handle multi input only in case of T1w, T2w, and dwi datatypes. Extend to func-task, func-regressors, and fmap. @giulia-berto

[soichih]

This is no longer the case, right? Can we close this?

[giulia-berto]

Hmmm, multiple fmaps still remain an issue. It's still unclear to me what we should do.