Third Party Classification support

[math-a3k]

Currently the way of adding another classifier to covid-ht is overriding get_current_classifier() to return an object that has the is_inferred attribute and implement the classification_tuple() - you may provide metadata in the classifier also.

It doesn't have to follow or done via the django-ai API - though you'll get its benefits if you do.

The requirement is that has to be integrated through Python in the server or instance.

The idea is to provide support for "external" classification via a REST API, so integration with a third-party classification services - i.e. written in R - is easily achivied.

[math-a3k]

Initial implementation in fb6fe9f - improvements in other issues