core: stdcm: reorganize stop data

[eckter]

Fix #8974

• A new class provides the stop locations to the InfraExplorer. The actual behavior doesn't change much, it's just to split the responsibilities. We'll eventually tweak this class to provide overtake stops. The intent is to eventually provide "optional" stops that generate two explorers (with and without), but that behavior isn't supported yet, to avoid cluttering this PR.
• InfraExplorer now provides an accessor to list stops locations on the path, though it doesn't keep track of any temporal data. The stop data itself isn't stored in the explorer, it's just an accessor for the underlying IncrementalPath.
• InfraExplorerWithEnvelope does not store anything itself (besides what's in the underlying InfraExplorer). It matches data from InfraExplorer stop locations and TimeData stop data to build the resulting envelope. No more redundant data there.

With these changes, it should be easier to implement proper overtakes. Safety speeds as well, though we'd need to be careful about lookahead. We need to know about the stop before entering the safety speed area.

---

EDIT: back to draft, there's some bugs around

So I missed some complexity:

1. We often refer to graph.steps to figure out if/when we should stop. That needs to go as well.
2. When the input says to stop at some given operational point, we may encounter several parts of that OP on the path. But we only need to stop once. Which involves keeping track of where we're at in the path.

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  Coverage   80.63%   80.63%           
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  Files        1101     1101           
  Lines      112322   112322           
  Branches      749      749           
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  Hits        90566    90566           
  Misses      21713    21713           
  Partials       43       43           

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