Separate parcel extraction from correlation workflow

[poldrack]

Is your feature request related to a problem? Please describe.
I have a use case where I would like to use nibs to extract betaseries averaged over parcels for use in another modeling application. Currently this requires using the correlation workflow, but because I am using a single event per beta, the correlation workflow returns an error.

Describe the solution you'd like
It would be clearest to separate the parcel extraction workflow from the correlation workflow. e.g. if the atlas information is specified then parcels should be extracted. if correlations are desired then there should be a separate flag.

Describe alternatives you've considered
N/A

Additional context
N/A

[jdkent]

Thanks for opening this issue, I agree, these are separate conceptual steps and should be separated.