Aggregate channels

[breedx2]

Relates to #15 First thing, rename the rollups to rollups so as to not confuse actually aggregation. Then, this adds a per-channel per-year aggregation file output that we create as part of the data generation GHA. In this file, we have:

• per-day totals of for each seen event
• year-to-date totals for each seen event
• list of most recent message of each type
• very most recent event name and timestamp