improve documentation of utils modules

resolves #502
I reviewed the utils modules to check for docstrings, and decided to import set_seed into the top-level API to make it easier to find

opensoundscape/__init__.py

@@ -31,3 +31,4 @@
 from .annotations import BoxedAnnotations
 from .preprocess.utils import show_tensor, show_tensor_grid
 from .localization import SpatialEvent, SynchronizedRecorderArray
+from .utils import set_seed

opensoundscape/ml/utils.py

@@ -169,10 +169,11 @@ def collate_audio_samples_to_tensors(batch):
 
 
 def check_labels(label_df, classes):
+    """check that classes and label_df.columns are the same, otherwise raise a helpful error"""
     class_err = """
             Train and validation datasets must have same classes
             and class order as model object. Consider using
-            `train_df=train_df[cnn.classes]` or `cnn.classes=train_df.columns` 
+            `train_df=train_df[cnn.classes]` or `cnn.change_classes(train_df.columns)` 
             before training.
             """
     assert list(classes) == list(label_df.columns), class_err