Leverage ASL derivatives?

[tsalo]

This would obviously be way down the road, but I was thinking rapidtide could leverage ASL derivatives (e.g., ATT or CBF) in some manner. It would be cool to feed in ASLPrep derivatives via the --derivatives flag at some point.

[tsalo]

Per chat with @bbfrederick, the ATT map is likely to be highly correlated with the delay map, but there will probably be a shift in values (i.e., the ATT values will be consistently lower than the rapidtide delay map).