Add `vetos` field to `EventSet`

It would be useful to have the veto mask for events computed up front, and it's a task that could be naturally parallelized during inference. I'm imagining it would be a `(num_events, num_ifos)`-shaped array, with each entry corresponding to the applicable vetos via a bash-permissions style encoding.

As part of this, we could turn the `vetos` functionality into a library.