Update BMI C++ header to match upstream with virtual dtor and correct namespace

[PhilMiller]

Cf NOAA-OWP/ngen#831 for the reasoning behind the virtual destructor inspiring the update.

The namespace change is to address the technicality that bmi::Bmi* is not actually equivalent to bmixx::Bmi*, even though the class definitions may be identical, and that difference technically entails Undefined Behavior when they're conflated in ngen.

Changes

• Update the definition of the base class Bmi to match upstream with its virtual destructor
• Use the same namespace bmi as upstream, and not a custom namespace bmixx to avoid UB

Testing

1. CI

Notes

• There's no automatic tests in ngen that reference LGAR-C/LASAM, so nothing will immediately break by merging this in an uncoordinated fashion. Having this work may require use of an updated ngen with Update both BMI C++ header copies to reflect virtual destructor ngen#832 integrated

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
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[hellkite500]

Rerun CI after dependent PR's are merged.