Track decisions IDs on LinkInfo, not Decision
There are a few rare cases where the same feature can be attached to multiple different nodes in the nav graph for the same chain.
The feature retrieval code would not add duplicate nodes, so features would only be associated to the first node.
This MR instead tracks the decision IDs on LinkInfo objects themselves so now all the legs should be included.
My test code now shows that the IDs seem to be correctly included.
FAO @tamartin