Removed duplicate string->path code from SubTree.
 Removed duplicate string->path code from Constraint.
Optimised ScenePlug::stringToPath().

This gives an 88% reduction in runtime for a synthetic test of just that function, and a 7% reduction in total runtime for a scene benchmark using the Instancer.
Merge pull request #1102 from johnhaddon/stringToPathOptimisation

Optimised ScenePlug::stringToPath()