Don't require 'roi' column, use roi-sets instead

[stuarteberg]

The first version of this tool exported a flat connectome and connectivity reports. Those assumed that the input data contained just one column for ROIs, named roi. Since then, the code has been altered to support multiple ROI columns, each referred to as an "roi set". The ROI sets need not be provided in the input synpase table; instead, they can be determined via ROI masks or segmentations in DVID.

But the original flat-connectome and reports functionality both still assume exactly one ROI column, named roi, and they don't use the user-defined roi-sets at all. We should change that somehow. Maybe the easiest thing would be to allow the config to name ONE of the roi-sets to use for the reports and.or connectome export.