Framework for description annotations

[cthoyt]

have a function that takes a namespace and a class to

def add_descriptions(namespace, annotator_cls):
   pass

and an abstract class to be the parent of all annotator_cls instances.

For example, this class might take care of caching as well.

class BelDescriptionAnnotator(object)
   @abc.abstractmethod
   def get_description(self, name):
      pass

keep in mind the difference between python2 and python3 abstract base classes