Symlink data directories into build directories

[program--]

This PR adds symlinks to the data/ and test/data/ directories in out-of-source build directories. This is done through CMake's file(CREATE_LINK ...) command.

Additions

• file() call to symlink <root>/data -> <build>/data
• file() call to symlink <root>/test/data -> <build>/test/data

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
• macOS

[PhilMiller]

This reduces the test failures I see in an out-of-tree build.

There's still other failures, but those can be addressed separately:

Can't init test_bmi_cpp; unreadable shared library file '{{EXTERN_LIB_DIR_PATH}}libtestbmicppmodel'

[PhilMiller]

(edited description to reflect my testing on macOS)