Deprecate implicit runner `init`s

[sneakers-the-rat]

Currently the initialization state can be implicit, calling process/run/iter without explicitly initializing the tube.

We should insist this is explicit:

as a contextmanager (preferred)

runner = SomeRunner(tube)
with runner:
    runner.process()
    # or
    for result in runner.iter():
        # ...
    # or
    runner.run()

or manually:

runner = SomeRunner(tube)
runner.init()
runner.process()
runner.deinit()