tweaks

some misc cleanup. Also added EventGenerator.pdf to evaluate the pdf encoded in the generators at the values associated with a particular event.

My basic idea was to split off the functionality that evaluates the pdf at the parameters stored within each event from the actual sampling. This way, one can sample from a generator even if that generator does not know how to evaluate its pdf.

I also changed the signature in EventGenerator.init with an eye towards moving all my stuff from exposure into a subclass as you suggested. I'm going to develop that as a separate branch (based on this one) and ping you when it's ready.