Adds InformationContentEvent

Updates the [ION Event Hierarchy](https://docs.google.com/drawings/d/1LXa33E_aBFeZxloegRQ7uMZ78lxhAmOGmL-wyMv83LM/edit).

- InformationContentEvent
- InformationContentModifiedEvent
- InformationContentStatusEvent

DatasetModifiedEvent extends InformationContentModifiedEvent