Theoretically, the bigger the value returned by the criterion, the closer the
current grouping is to the best possible grouping.
Each CRITERION internally defines two functions. One is used to
evaluate if moving a vertex from one group (community) to another
possibly yields a better overall result. The other is used to measure
the actual overall quality of the entire grouping (current community
mapping).
Not all criterion might be available for all algorithms. See each algorithms'
help to find which criterion is supported