Misc cleanup: 'using namespace' and unnecessary 'friend'

[PhilMiller]

Just clearing out some poor form C++ that I noticed

Changes

• Eliminate using namespace models::bmi in headers
• Eliminate using namespace hy_features::hydrolocation; in header
• Delete unnecessary friend class Bmi_Multi_Formulation
• Shift a header include from header where it wasn't needed to source files that actually depended on it

Testing

Code still builds

Checklist

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