get rid of Entity subclasses

[mscarey]

Designating an object to be a member of an Entity subclass such as Association, rather than just an Entity, seems to be equivalent to creating a Fact that "X was an association", so there will still be a way to accomplish the same thing if the feature is removed.

But if it stays in, I don't know how to represent the concept that Rule A applies to Association X notwithstanding the fact that Rule A was about a regular Entity rather than an Association.

In the Introduction notebook, I wasn't able to do this:

nosferatu_rule = lotus_majority.holdings[0].new_context(
    {Association('Borland International'): Human('F.W. Murnau'),
    Entity('the Lotus menu command hierarchy'): Entity("Dracula")}
)
assert nosferatu_rule.means(lotus_majority.holdings[0])

And I can't think of any reasonably easy way to do it.

[mscarey]

These subclasses are removed in 8f864229cf5109fc9e5f7c9ec6aa7304c71d2458.