Optional locations, potentially with penalty

We should expose an option to configure locations as being optional.
Potentially adding a penalty to the solution if the location is not serviced.

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Implementation: see `model.AddDisjunction()`

Start by reading the existing binding around model.AddDisjunction() and how routing options are exposed. Clarify the API for marking a location optional and for applying a penalty, then verify that both behaviors are represented in the relevant routing examples or tests; done means optional locations can be skipped and penalties affect the solution as specified.

Written by the indexing model from the issue text.