Helpers should be in a SCL class

Helpers are currently attached to the ocl:String class (and one to the ocl:Integer class). This is OK with CompleteOCL, but KO if we want to merge OCL constraints in the SCL Ecore file. The latter is needed to get the improved performance of OCL validation with OCLinEcore compared to CompleteOCL (and the OCL to Java conversion which currently is not available with CompleteOCL).