fix ParallelContainer's interface methods

[rhaschke]

A parallel container's required interface is derived from the required interfaces of all of its children.
They must not conflict to each other. Otherwise an InitStageException should be thrown.

• Implement unittests in core/test/test_container.cpp, following the example of SerialContainer, to validate correct initialization of allowed (and failure detection for forbidden) combinations. The following combinations are feasible:
  • ❘ ❘ = ❘ (connecting stages)
  • ↑ ↑ = ↑ (backward propagating)
  • ↓ ↓ = ↓ (forward propagating)
  • ↑ ↓ = ⇅ = ⇅ ↑ = ⇅ ↓ (propagating in both directions)
  • ↕ ↕ = ↕ (generating)
• requiredInterface(): correctly derive a consistent interface from children or throw an InitStageException. Not all combinations of child interfaces are possible.
• init(): use ParallelContainer::requiredInterface() to determine the interface
• implement pruneInterface(): forward pruning to all children with UNKNOWN required interface, follow SerialContainer's implementation
• fix validateConnectivity(): an exact match of ParallelContainer's interface to all children's interfaces is not required. ParallelContainer's interface, only needs to be a superset. For example, children interfaces of type ↑ or ↓ match to container's ⇅, but not vice versa.

[rhaschke]

Fixed via d731e94.