label_fn should do something reasonable

[JaimieMurdock]

the label_fn command arguments are undocumented and super obtuse. They take the corpus's metadata object, which may not be the intent of the programmer. This should just be a function that takes a doc id and returns the label - if you use the metadata in the corpus great, but the option should be available to just use a simple dictionary or lambda.

v.doc_entropies(label_fn=lambda x: folder_md[x]['citation'])