Add feature to calculate true event rate

[seaflow-uw]

Add feature to calculate true event rate

[ctberthiaume]

Actually, while it would be possible in theory to only read the first four bytes to efficiently get the event count, it adds significant complexity and I'm not sure it's a net positive. All files are gzipped, so just getting the first 4 bytes isn't completely straight-forward. And the method of reading from object storage would need to be modified to support byte range requests. For now just read the entire EVT file as normal and use the row count.

[ctberthiaume]

See e7d1e74 for this feature.