Grouping faults with different descriptions messes up their descriptions

[lkev]

The output description of the grouped events from get_grouped_event_data function of batch.py works fine when the different events to be grouped together have descriptions that only differ by one thing, e.g. an axis number.

However, when they are different, the output description gets mangled.

Should be a way to override the default behaviour and include a custom output description for these events.