expanded EventContext and EventIDBase

the EventContext class needs to be expanded in two ways

• bring in event identifying information (event#, run#, timestamp, lumi#, etc)
• some experiment dependent ptr for other stuff

in the case of ATLAS, the latter is an IProxyDict ptr, which probably means nothing to someone not on ATLAS. I kept it as such in the code as an example. I think a more neutral way of doing this would be just a void* ptr. We could also have a small array of void* ptrs, or even a vector of them, but for the moment, I think that's overkill. I don't, however, think we want to go the route of having various experiments inherit from a base EventContext class, as that will introduce dcasts everywhere.

for the EventID stuff, I offer a simple EventIDBase class that's a subset of what's in ATLAS's EventID class, but which I think will be experiment neutral. I am not beholden to any of the method names - this was just copied from the ATLAS repository - but keeping them as such would save us having to re-implement them ;-}