introduct Concept class (and subclasses?)

[sbenthall]

Introduce Concept class as wrapper around a semantic node. A Concept has:

• a set of terms that elicit it (e.g. a single term, or set of words in a WordNet synset)
• a reference to the knowledge base they are a part of (see Ned Block on Conceptual Role Semantics)
• a method that returns the available relations for this concept, in the knowledge base and/or based on wordnet (antonyms)

[sbenthall]

With this, there are new performance reasons for using this:
8b9abdf

We can achieve O(1) time checking a relation against a knowledge base. This is necessary for large knowledge bases. But when testing for relation equality means testing for synonyms or antonyms, that means we can't hash on strings for the internal data structure of Doxaments.

So, Concepts need to be hashable, and their antonym Concepts (consistently hashed) need to be hashable as well.

This could lead to problems, as right now (because we aggregate lemmas across many word senses), it's is not the case that if anto(x,y) then anto(y,x). Hmm

[sbenthall]

See

bc63dd6

for performance improvements without implementing the Concept class. Could probably squeak by without this functionality to get a 0.1 out sooner.

[sbenthall]

Bumping to 2.0, may need to be nixed as we redesign parsing and doxament structure.