I think this check should be placed in ```Interface._validate_outputs```. This function currently isn't doing anything (just ```pass```ing). It's a good place for the check because it'll happen when the interface is constructed but before any computation is performed (think of it as a config file validation step). I think it makes sense for this check to go in the ```Interface``` baseclass because it isn't really specific to optparse.

Also, while looking through that file, I noticed that many of the docstrings still refer to the old class names (e.g., ```InterfaceOutput```). Can you please take a pass through the file and update the docstrings when you make this change?