Attempt to correctly partition in the presence of a terminal nexus with no defined outflow destination (NGWPC-6553)

[PhilMiller]

Changes

• Network class: generate sentinel water-body vertices as outflow destination features for nexuses that would otherwise not have a downstream catchment present in the job
• partitionGenerator: Use a count of catchments after the above sentinel vertices have been added, rather than the number in the specified subset, to not be thrown off in partition-sizing by the potential additions

Testing

Notes

Todos

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Testing checklist (automated report can be put here)

[PhilMiller]

Various Network unit tests do not like that there's added sentinel node(s), unsurprisingly. I'll work out whether to tweak the construction logic, the indexing/enumeration logic, the tests, or some of each.

[PhilMiller]

Closing in favor of #32 which does not involve changing the implementation of Network, and hence doesn't suffer the same challenges with its tests.