Symmetric

[cbizon]

• Fixes two bugs, one returning a None when a [] is better, and one indentation
• Optimizes the enrichment calculation a little bit by checking the fail conditions before doing calculations
• Partially modifies the enrichment to handle the fact that symmetric edges are only stored in one direction

Things still to do:

• The symmetry handling is looking only for related_to, it needs to be expanded to all symmetric predicates. This probably means bringing in BMT, and creating a set of symmetric predicates at startup time, then checking against that instead of just looking for related_to

[cbizon]

Also need to fix the tests. This may relate to whether or not to filter on the number of input nodes. For normal AC we often want to do that, but then we also may not for the prototype-style enrichments. So we probably need to make that an option on the function.