Fix generateTributaryMaps

[slifty]

There was

Issue #95

Description

This PR fixes an issue where generateTributaryMaps would sometimes throw an error if a populated streamOutputMap did not contain a valid output for every input of a given station.

Due Diligence Checklist

• [x] Tests
• [x] Documentation
• [x] Consolidated commits
• [x] Relevant labels assigned
• [x] Changelog(s) updated

Steps to Test

1. yarn test

Deploy Notes

None

Related Issues

Resolves #95