Is your feature request related to a problem? Please describe.
Currently vak.io.dataframe.spect_output_dir defaults to data_dir

This seemed like a logical choice at the time but it has the nasty effect of making tons of sub-directories full of .npz files in a directory you might not be paying much attention to.

For example, just had to fix this happening with tests: #571

Describe the solution you'd like
A better choice might be to default to output_dir? Since this is a directory we will be paying attention to as we work with models.

Describe alternatives you've considered
(1) Keep things as they are.

(2) And maybe have output_dir default to data_dir inside cli.prep? '
I.e., the default is to have spectrogram files to live besides the audio files they were generated from, unless a user really wants them to go somewhere else.

The drawback to this is that we then have a default that can easily overwrite existing spectrogram files, which we may not want to do. And those spectrogram files might be expensive to generate, depending on the algorithm and data!

Could make this opt-in, i.e. by specifying the same dir as data_dir for output_dir