Record and avoid duplicates in merging

[OwenArnold]

This ticket is blocked by :

• TRAC9257

This ticket is blocks : TRAC9509

One of the problems with parallel implementations of CCL (http://trac.mantidproject.org/mantid/ticket/9257) is that when tracking cluster across boundaries, sets of labels may be duplicated many times. This means that unnecessary merge operations are performed in ClusterRegister::merge.

It should be faster to track pairs that have already been merged. To do this, you need to implement a hash operation so that a, b can be treated as equivalent to b, a. boost::hash probably provides the answer here. Once you have an appropriate hash operation, you can track pairs in a set, and only process pairs that have not already been identified.

[OwenArnold]

This issue was originally trac ticket 9360