Conditionally run candidates

[Joe Alcorn]

Maybe we don't want to run the candidates on every invocation. We should have a way to skip candidate execution.

Similar to the way scientist has implemented this, I'm thinking something like this for the API, which would enable it 10% of the time

class Experiment(laboratory.Experiment):
    def enabled(self):
        return random.choice(range(1, 100)) > 90