Add missing override specifiers to quiet warnings

[PhilMiller]

CI builds using Clang produce warnings like

[ 44%] Building CXX object src/bmi/CMakeFiles/ngen_bmi.dir/Bmi_Cpp_Adapter.cpp.o
In file included from /Users/runner/work/ngen/ngen/src/bmi/Bmi_Cpp_Adapter.cpp:1:
/Users/runner/work/ngen/ngen/include/bmi/Bmi_Cpp_Adapter.hpp:296:18: warning: 'is_model_initialized' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
            bool is_model_initialized();
                 ^
/Users/runner/work/ngen/ngen/include/bmi/Bmi_Adapter.hpp:35:26: note: overridden virtual function is here
            virtual bool is_model_initialized() = 0;
                         ^
1 warning generated.

They're not wrong.

Changes

• Add the missing specifiers

Testing

1. Compilation with AppleClang no longer emits warnings

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 ➡️

Target Environment support

• Linux/gcc
• macOS/AppleClang
• Linux/clang