Add option to vectorize connectivity matrices

[tsalo]

Summary

@tjhendrickson requested this for a future HBCD release. I would prefer for vectorized connectivity matrices be produced as an alternative to the square versions, rather than as additional outputs, but it's ultimately up to the HBCD stakeholders. I started on this in #1308, but since it's a low-priority item I'm going to close that and track the request in this issue.

I'm guessing that the requested organization will not match the sparse format from the BIDS-connectivity BEP(s).