* Memory leak in .NET user-propagator
The user-propagator object has to be manually disposed (IDisposable), otherwise it stays in memory forever, as it cannot be garbage collected automatically
* Throw an exception if variable passed to decide is already assigned instead of running in an assertion violation
* Added limit to "visit" to allow detecting multiple visits
* Putting visit in a separate class
(Reason: We will probably need two of them in the sat::solver)
* Bugfix
