change find_events to accomodate neuromag bug

I have changed the defaults for find_events. This sets the default min_duration to 2 samples. This is useful, because a common Neuromag system "bug" exists which can result in the first sample of an event having a value less than the true trigger value (due to the combined binary trigger numbers). e.g. a trigger of 17 can have one sample of 16 prior to the real trigger value of 17.