`hvc.extract` not saving features file when called with arguments, not config file

[NickleDave]

high-level hvc.extract file does not save a features file when called with keyword arguments (as opposed to passing it a single config file)

noticed this when going through the tutorial
https://github.com/NickleDave/hybrid-vocal-classifier-tutorial/blob/master/notebooks/02-feature-extraction.ipynb
raised an issue there too
NickleDave/hybrid-vocal-classifier-tutorial#6
but I think it's the code itself, I could reproduce the bug just running in ipython directly from this repo

[NickleDave]

yes, definitely the case that nothing gets saved in the directory passed in as an argument for output_dir even though the tutorial says otherwise

if return_features is True though, a dict is returned with keys features and labels