made a small change to the cluster module that allows for heuristics to be provided as a string name of one of the provided heuristic functions instead of the function itself. Also caught a doc typo.
made a small change to the cluster module that allows for heuristics to be provided as a string name of one of the provided heuristic functions instead of the function itself. Also caught a doc typo.

cmaclell/concept_formation,cmaclell/concept_formation,cmaclell/concept_formation