Delete unused stub class HY_HydroNetwork

[PhilMiller]

The class HY_HydroNetwork has no substance, nor any uses. It contains a spurious reference to a collection of HY_CatchmentArea instances, which threw me off in other refactoring work.

Removals

• class HY_HydroNetwork

Testing

1. CI

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)
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[PhilMiller]

There was a bit of discussion that we may want to keep something like this around for future modeling needs. However, right now, the code that's present supports nothing, and would be pretty trivial to resurrect or reproduce