protect user data from garbage collection
protect user data from garbage collection

 - add member `_stuff` to `Model`
 - add all constraint handlers and constraints to it
 - TODO: needs to be done for all plugins!

mattmilten/PySCIPOpt,mattmilten/PySCIPOpt,SCIP-Interfaces/PySCIPOpt,SCIP-Interfaces/PySCIPOpt