Fix warnings from -Wsuggest-override

[PhilMiller]

We want to be able to build the code as strictly as possible without compiler warnings. This fixes warnings from -Wsuggest-override, which indicates the potential for hidden mistakes relating to dynamic method dispatch, and hence incorrect results.

Changes

• Add override on various member function declarations throughout the code and tests
• Update Sloth with https://github.com/NOAA-OWP/SLoTH/pull/6
• Add -Wsuggest-override to CI build with -Werror so new instances aren't introduced

Testing

1. CI passes with no instances of the offending warning

Notes

• Once https://github.com/Unidata/netcdf-cxx4/pull/152 is merged, we can update our submodule of netcdf-cxx4. In the meanwhile, it's not used in CI, so there's no failure from it.

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: