Party.lag_calc unclear what happens if stream does not cover all detections

**Is your feature request related to a problem? Please describe.**
`Party.lag_calc` takes a stream, but it isn't clear what would happen if that stream did not encapsulate all detections.

**Describe the solution you'd like**
Document the expected behaviour, and if that is that it breaks if insufficient data are provided, change that so that it warns that data are not there and continues.