Better handling of nullary predicates

[daveraja]

Nullary (0-arity) predicates don't really fit the ORM model. Currently, it is possible to declare nullary predicates in clorm by simply not specifying a parameter to the predicate class definition.

  class P(Predicate): pass

But this creates a predicate sub-class only to match a single proposition "p" and every python instance of P will match the same predicate. If you have a lot of nullary predicates in your ASP program then you have to create a class for every one of them. This is very heavy handed and doesn't make it very useful to integrate into clorm queries.

So we need a better way to deal with nullary predicates. Currently, I don't have a clear idea. Maybe create a special case NullaryPredicate class to store arbitrary nullary predicates (eg., "p","q","r","s", ...). But integrating it into the factbase query language might still be very hacky.