Vak

Added
- `vak.cli.summary` has `save_transformed_data` parameter and `vak.cli` passed value from
`config.data.save_transformed_data` as the argument when calling `vak.cli.summary`

Changed
- `vak.cli.summary` only saves transformed train/test data if `save_transformed_data` is `True`
- move a test from tests/unit_tests/test_utils.py into tests/unit_tests/test_utils/test_data.py

Removed
- `vak.cli.summary` no longer saves copy of test data in results directory

Added
- add test for utils.data.get_inds_for_dur

Changed
- learncurve gets indices for all train data subsets before starting training

Added
- Use `attrs`-based classes to represent sections of config.ini files

Changed
- rewrite `vak.cli` so it can deal with state of config.ini files
+ e.g. doesn't throw an error if `train_data_path` not declared as an option in [TRAIN] when running `vak prep`
(since training data won't exist yet, doesn't make sense to throw an error).

Removed
- remove code about `freq_bins` in a couple of places, since the number of frequency bins
in spectrograms is now just determined programmatically
+ `vak.config.data` no longer has `freq_bins` field in DataConfig namedtuple
+ `make_data` no longer adds `freq_bins` option to [DATA] section after making data sets

Fixed
- add missing 'save_transformed_data' option to Data config parsing

Changed
- checkpoints saved in individual directories by `learncurve` so they are more cleanly segregated,
e.g. if user wants to point to a specific checkpoint when calling `predict`
- calling `vak prep config.ini` will run `vak.cli.make_data` function
+ so to generate a learning curve, the three steps now are:
bash
vak prep config.ini
vak learncurve config.ini
vak summary config.ini


Fixed
- `vak.cli.train` runs all the way through, passes basic "does not crash" test
- `vak.cli.predict` runs all the way through, passes basic "does not crash" test