[DRAFT] Generalized ThreadHierarchy & removed ScopeHierarchy

[ThomasHaas]

No description provided.

[ThomasHaas]

Description: ThreadHierarchy is class that already existed and was used to define hierarchical scheduling. It was computed from the ScopeHierarchy associated to threads.
The idea now is to make ThreadHierarchy the canonical container for all threads in the program, i.e., Program directly manages a ThreadHierarchy. Whenever a Thread is added to the program, it will have to get inserted into the hierarchy.
The hierarchy has, by default, a root node with scope GLOBAL and a INIT node containing all init threads, even for CPU programs (LLVM/Litmus).