Monte carlo

Here's my idea for getting the monte-carlo stuff out of exposure and into gw_event_gen. Basically, I've declared a child class similar to what you did in !7 (closed), but with slightly different functionality and syntax. There are 2 big differences

• my implementation requires the user to declare a set of required generators and transforms before instantiating MonteCarloIntegrator(EventGenerator). I also manage a few of the internals slightly differently.
• I'm going to move all the I/O methods you had under EventGenerator into stand-alone functions that read/write lists of Events from/to disk. These can then be called in simple delegation schemes from EventGenerator's classmethods (if desired), but I think we'll be better served long-term with these separated out. There are operations, for example, that one can perform on a list of Events that do not require a full EventGenerator and I'm inclined to allow users to access lists of Events without ever going through or knowing about EventGenerator objects.