Qt interface doesn't have any indication which trains contain data

[takluyver]

If you open a run where the first few trains have no detector data, there's no easy way to find useful trains to draw - you just have to go through until you find one with data.

The notebook template handles finding the first train with useful data.

[takluyver]

This is improved by #20, which drops empty trains at the beginning/end of a run. It would still be useful to be able to 'hop over' missing trains, e.g. if the new pause/resume feature is used. The spinboxe currently just increments/decrements the number and then tries to retrieve that train.