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

[calum-chamberlain]

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.