Define ROI subsets to analyze differently

[swkeemink]

FISSA currently also does not have an easy way to only extract the traces for certain subROIs (i.e. without doing the whole neuropil region extraction etc.), might be worth implementing that.

This could be useful if users want to record e.g. a global background signal, or some local reference signal, or just want to extract the signals from some ROIs without the surrounding neuropils.