ENH: Add ability to specify function/callable to make spectrogram during prep step

[NickleDave]

To be able to replicate results from some models, e.g. Sainburg et al 2020 and Goffinet 2021, it would be good to use the exact functions they use to compute spectrogram. This could also be helpful for users more generally.

We should achieve this by allowing a user to specify the callable / function to use to make spectrograms.

I imagine in a config this would look something like

[vak.prep.unit_dataset]
spect_maker = 'avgn.signal_processing.spectrograms.make_spec'
[vak.prep.unit_dataset.spect_params]
nperseg = 512
noverlap = 64

Where spect_params can be any kwargs that are passed into the Callable specified by spect_maker

Under the hood we will need to do

• make sure we can import the callable
• document what makes it possible to do so, i.e., have it installed or have it in the current working directory
• determine dynamically if it's a class we need to instantiate -- can we do this? I think yes, with isclass
• pass spect_params into the spect_maker

We should probably do this at the same time we switch to using vocalpy as a backend in the prep step as in #558