Add attribute with list of waveform/PE sample keys for each input event

In addition to the events attribute, it'd be super useful to have another attribute listing the specific waveform/PE run used in tandem with each event. This could be a distinct attribute, or the events attribute could itself be a list of tuples, where each tuple is an (event name, PE samples) pair.

If others agree that this'd be useful, I'm happy to work on adding this!