Method-based Contract Composition

Following Features added to FeatureHouse:
  - Conjunctinve Contract Refinement as Composition Mechanism for JML-Contracts
  - Cumulative Contract Refinement as Composition Mechanism for JML-Contracts
  - Method-based Composition:
         - you can use a keyword to specify which contract composition mechanism should be used for a Contract Composition. This keyword may be refined.
        - keywords: \final_contract (Plain Contracting), \cumulative_contract, \consectuive_contract, conjunctive_contract, \final_method (Refining Contract and Method is not allowed)
              Explicit Contract Refinement (Contract Overriding) does not need a keyword
  - CompositionException if something went wrong during contract composition