WIP: Custom data refs and related changes to the new testing framework

[mbrachner]

This pull request adds custom data refs to XPNet. This brought also a breaking change to the new testing framework by @jaurenq when it comes to the use of the test harness. For all datarefs (not just the custom datarefs) setter and getter functions are needed, but there are some predefined methods in XPMock, that make the handling easy and straight forward.