Adapt attention allocation MindAgents to use IndefiniteTruthValues

[ferrouswheel]

Currently only the mean of TruthValues is used by the AttentionAllocation system. It's be nice to optionally use IndefiniteTruthValues in some way. Both in updating the weights of HebbianLinks and in calculating the amount of STI that is spread across them.

(This would involve some research into the best way to represent this however)

This is on Joel's list of things to do (in the far future), but if others want to play with the problem too then collaboration would be gladly accepted.