Global variables in a Singleton

[jbm]

Just make sure, you do not violate this one:

NOTICE: Behavioral change for isDefined and getTitle

In the PolyScope release of March 2019, we will perform a behavioral change in the URCaps API. The change will not break compatibility with the API itself, but the behavior of some methods are affected, which can cause Exceptions to be thrown in some cases. The implementation of the two overridden methods isDefined() and getTitle() in the Contribution-class has the purpose, of helping the user understand the purpose of the node, and whether this requires more data, to be valid. The methods ar…