Change offset axis to actual bins not category?

Right now the offset axis for IRFMetrics is a Category axis, but since they are actually bins, it doesn't make sense to not just use a Regular axis. That will allow plotting e.g. along the offset direction, which we do want to have, e.g. for a given energy, plot the angular resolution as a function of offset.

I'm not quite sure if there was a good reason to use Category though.

It would also solve some plotting issues.