Loosen dependency on netcdf-cxx4 to 4.3.0 from 4.3.1

[PhilMiller]

There was an API change that's not backwards compatible from 4.3.0 to 4.3.1, in adding a member function. Avoid using that member function to gain compatibility with the older release.

Changes

• Use C API free function nc_free_string from base netcdf instead of member function freeString from separate C++ bindings in netcdf-cxx4

Testing

Modified version used in benchmark/scaling runs on Hera.

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 ➡️

[PhilMiller]

This should probably be done independent of whether we vendor in netcdf-cxx4 (cf #729), since we'll presumably still use an external copy if it's part of the base netcdf installation