BindingSiteConstraint bugfixes
1. site_constraint is an orphaned ScoreType. BindingSiteConstraint is a functioning constraint that uses atom_pair_constraint as its ScoreType, but optionally uses whichever ScoreType you request. I have added site_constraint to the list in ConstraintsEnergy so that you may use site_constraint with BindingSiteConstraint.
2. BindingSiteConstraint's Tracer was mislabled (copy paste error)
3. BindingSiteConstraint's clone() function was missing an argument in its dispatch to the copy ctor and thus forced cloned copies into the default ScoreType instead of an assigned ScoreType.