- Device subclasses should provide their own implementation.
Default behaviour is to raise a `NotImplementedError`
- Workload subclasses can set `requires_network` to `True` and
network connectivity check will be performed during `setup()`
- standardisded on a single context argument
- removed Device.init() no longer necessary as initilize now
automatically gets propagated up the hierarchy. Renamed the existing
use of it to "initilize".
- related pylint cleanup.
- added initialze and finalize methods to workloads, which were the only
major extension types that did not have them
- Semanatics for initialize/finalize for *all* Extensions are changed so
that now they will always run at most once per run. They will not be
executed twice even if invoke via istances of different subclasses (if
those subclasses defined their own verions, then their versions will
be invoked once each, but the base version will only get invoked
once).
