#1222: make use of homotopy configurable in the global system object

- Utilities.mo: introduce new function homotopic, making use of homotopy configurable
- System.mo: introduce configuration parameter use_homotopy
- Fittings.mo, Machines.mo, Pipes.mo, Valves.mo, Vessels.mo: replace calls to homotopy with calls to Utilities.homotopic
- TestPipesAndValves.LumpedPipeInitialization: adjust configuration of valve to DynamicPipeInitialization


git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6885 7ce873d0-865f-4ce7-a662-4bb36ea78beb