Make Event._event public or provide mapping of event names to nametuples

[moloney]

Description

Most of my code uses the NotificationEvent / InterventionEvent namedtuples (e.g. as dict keys) since that is the required input for most of the pynetdicom API (i.e. the "bind" methods). However when we get an Event instance in a callback, only the event name is a public attribute.

It would be nice in the private '_event' attribute was made public (although perhaps it needs a different name), or if there were some mapping provided to lookup the namedtuples by the event name. This isn't hard to work around, but figured it would make the API a bit more consistent and ergonomic.

[scaramallion]

Sounds reasonable