Refactor: A SkeletonNode should have more than one 'Pivot'

[dbinkele]

The method 'getInstantiatedSubtree' in 'IVariabilityAwareArtifact' currently returns a 'Pivot'-Object. The goal is to add a second version of this method which returns a collection of 'Pivot'-objects. Each 'Pivot' is wrapped in a new Pair-Object, which additionally contains a a node<> 'n' on whose presence the corresponding 'Pivot' depends. Further code will only use the version 2 of the method as we deprecate the old one. We guarantee backwards compatibility by providing a default implementation for v2 which calls v1. It returns a pair <n, Pivot>.

In 'iest3.os' there are two code locations where the method is called:

1. In http://127.0.0.1:63320/node?ref=r%3A211c2313-6a94-46b9-b5b0-2442332ff5ac%28org.iets3.variability.artifacts.base.behavior%29%2F234807277263800793 the mehod 'artifactPathFor'. This method is not used... deprercate it .
2. http://127.0.0.1:63320/node?ref=r%3A211c2313-6a94-46b9-b5b0-2442332ff5ac%28org.iets3.variability.artifacts.base.behavior%29%2F2304805897528296615 ... this method returns an 'ReferenceResolved'-Object. We must change it to a collection of 'ReferenceResolved'-Objects.