Speed up actions in coercion model

[jdemeyer]

This is the logic that the coercion model uses when it calls an action:

1. Get the action object (an Action instance) given the parents of both arguments and the operation. This is cached using TripleDict, so lookup of known actions is fast.

2. Call the action using the generic __call__ interface.

3. The action checks again the parents of the arguments.

4. The actual action method _act_ is called.

Step 3 takes a non-trivial amount of time and is completely unneeded since the coercion model already knows that the parents are correct.

We also remove the (op is not sub) and (op is not isub) checks: I see no reason why actions should not be allowed for subtraction.

[jdemeyer]

Never mind, the description is wrong.