bug: WindowDataset.crop_spect_vectors_keep_classes method can crop twice

[NickleDave]

as spotted by @yardencsGitHub
in line 312, the variable lbl_tb_cropped should be just lbl_tb -- not the vector that's been cropped already. As currently written, this would have the effect of "cropping" the dataset twice
https://github.com/NickleDave/vak/blob/68bba5865436ec7d705fdca82b4ee021dc86bf81/src/vak/datasets/window_dataset.py#L312

    # try cropping off the end first
    lbl_tb_cropped = lbl_tb[:cropped_length]

    if np.array_equal(np.unique(lbl_tb_cropped), classes):
            x_inds[cropped_length:] = WindowDataset.INVALID_WINDOW_VAL
            return spect_id_vector[:cropped_length], spect_inds_vector[:cropped_length], x_inds
        else:
            # try truncating from the back instead
            lbl_tb_cropped = lbl_tb_cropped[-cropped_length:]  # <-- SHOULD BE `lbl_tb` !!!
            if np.array_equal(np.unique(lbl_tb_cropped), classes):
                x_inds[:-cropped_length] = WindowDataset.INVALID_WINDOW_VAL
                return spect_id_vector[-cropped_length:], spect_inds_vector[-cropped_length:], x_inds