Convert mods/datums/ from tracked files to a git submodule pointing
to https://git.kindred-systems.com/kindred/datums.git (branch: main).
Follows the same submodule pattern as silo, solver, and gears.
The datums remote repo was initialized from the existing tracked files
with the package.xml repository URL updated to point to the new repo.
CMake install targets in src/Mod/Create/CMakeLists.txt continue to
work unchanged since the files remain at the same paths.
