Build modules under extern in the build tree, not under the source tree

[PhilMiller]

For everything under extern/, both Git submodules and support libraries, move their build output below the main CMake binary directory, rather than generating a cmake_build directory under each of them. This avoids issues with the compilation being mis-matched in various ways from the main framework build.

This comes at the small cost of duplicative compilation, but I think it's worthwhile, given the frustrating and confusing failure modes that arise from the noted mismatches.

Changes

• Various CMakeLists.txt to redirect where they are ultimately building things
• Tests with hardcoded paths to the relevant BMI modules
• Realization configuration files used in testing and examples to update BMI module paths

Testing

Screenshots

Notes

Todos

• Update documentation and other bits to match.

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 ➡️

Testing checklist (automated report can be put here)

1. Local ctest
2. Github CI jobs

Need to look more closely at failing tests not finding libs in new paths