Improve the abstraction of external data

Currently, Minerva includes the (poorly utilized) SaveData event type. This should be abstracted to allow for more general interaction with the external system. Perhaps there should be a ReadData as well as a save data type (although the utility of this is debatable). ListenForTrigger and ExternalTrigger event types have obvious utility on an Arduino node as well as a full stack computer, so perhaps we replace the SaveData event type with these instead.

Affected modules:
- event
- all modules that use event (most modules)