Add heuristic based user obejct for clustering

[magnoxemo]

Reason

For the mesh tally amalgamation we need to level the element that they are part of a cluster for which I am gonna use the extra element integer for elements. We need to create different heuristic based user object based on which clustering of two adjacent side / face sharing elements can be clustered.

Design

There should be a ClusteringHeuristicUserObejctBase class which will create and set up the common functionality and metric variable for the other heuristic user objects. It will not implement mesh walking process.

There should be another user object BooleanComboClusteringUserObejct which will implement the mesh walking process and evaluate a boolean expression based on which if they are part of cluster is decided.

Impact