Implement ConstraintMatrix::merge() for constraint matrices localized by an IndexSet (same IndexSet on both CM).
Implement ConstraintMatrix::merge() for constraint matrices localized by an IndexSet (same IndexSet on both CM).

git-svn-id: 31d9d2f6432a47c86a3640814024c107794ea77c@23867 0785d39b-7218-0410-832d-ea1e28bc413d

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