New maxNrAssignment scheme for pruning

[varunagrawal]

As discussed in our meeting yesterday, this PR updates the DecisionTreeFactor::prune method to use the new maxNrAssignments scheme.

Here, we use the record of assignments for each leaf to compute the number of duplicate values in the tree without enumerating all the (exponential) number of assignments, and threshold on the top maxNrAssignments values. To facilitate this, I added a new visitLeaf method to DecisionTree which follows the same idea as visit in that we visit all the leaves (and not assignments), but visitLeaf passes the full leaf object to the functional, allowing us to do leaf.nrAssignments() and leaf.constant() quite easily.

NOTE I have one question: should the functional receive the leaf object or the leaf pointer? It is a const either way, but I've made it as a leaf object const-ref so there shouldn't be a memory hit and it is convenient to use (since we don't declare a LeafPtr typedef).

[ProfFan]

@varunagrawal Shall we merge this?

[varunagrawal]

I'm waiting for approval on #1155