Adding warning for no injections

[aleynaakyuz]

This raises a warning if no injections are applied during an inference job.

[ahnitz]

@aleynaakyuz I don't think this will do what you think. I think you'd only see a message if the injection-file option wasn't given. I think what you need to do is to add a logging message into the InjectionSet class itself where the injections are applied.

[ahnitz]

Think about potential changes here. https://github.com/gwastro/pycbc/blob/master/pycbc/inject/inject.py#L561