Isolate sub-slot data from the parent slot

In the past it was useful for sub-slot algorithms to access data in the parent slot by default.

Increasingly we find that this causes more problems than it solves, with unexpected interactions between algorithms that access data objects with the same name but different (intended) content.

Since this behaviour was originally an addition to the "natural" scheduler dependency resolution, I have simply removed it again. See GaudiHive/src/PRGraph/Visitors/Promoters.cpp

I have also created a test to ensure that the isolation is working