Behaviour refactor

[mjp41]

This separates behaviour into two parts:

• BehaviourCore that handles the dependency management

• Behaviour that handles the precise instantiation and lifetime management.

The core aim is to allow BehaviourCore to be reused by a zero allocation notification mechanism. This would have different lifetime management, than a standard behaviour, and would require different code to run when the behaviour is invoked.

[mjp41]

Individual commit move in steps to try to preserve history/blame