EventContext: make constructor explicit
We do not want to implicitly convert an integer to an EventContext as
this would almost certainly result in an invalid event context.
We just had a case in ATLAS where something like the following code compiled fine and only at runtime we got an invalid EventContext error:
void foo(const EventContext& ctx);
int context; // variable incidentally named context that has nothing to do with an `EventContext`
foo(context);