GLM plotting should use groups, not events, in time height plot

[deeplycloudy]

Since all events in a group occur at the same time, only the group time is needed.

This requires filtering the whole dataset to those groups (and flashes) that have events within the current view. The filtering process is also related to #50, so it would be good to solve that issue at the same time.

I've started a canonical "show me all the data near the WTLMA" example in deeplycloudy/lmaworkshop#2. I'll use that as a way of demonstrating how to do the implementation.