Add support for MOM6 stochastic_physics

[mathomp4]

This pull request adds support for building and linking the external stochastic_physics package in the MOM6_GEOSPlug project. If the external package is available, it is used instead of the internal compile-only stubs; otherwise, the build falls back to the internal stubs with a warning. The build system is updated to handle this logic, and a new CMake configuration for the external package is included.

Build system enhancements for stochastic physics:

• Updated MOM6_GEOSPlug/CMakeLists.txt to detect the presence of the external stochastic_physics package and conditionally add its subdirectory; sets a flag MOM6_USE_STOCHASTIC_PHYSICS and provides a warning if the package is not found.
• Modified MOM6_GEOSPlug/mom6_cmake/CMakeLists.txt to only include the internal stochastic_physics source files if the external package is not present, and to add the external package as a dependency when available. [1] [2] [3]

Addition of external stochastic physics build configuration:

• Added new MOM6_GEOSPlug/stochastic_physics_cmake/CMakeLists.txt to build the external stochastic_physics library, specifying its source files, dependencies, and required compile flags.

.gitignore update:

• Updated .gitignore to exclude stochastic_physics directories, matching the handling of the mom6 directories.

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Closes GEOS-ESM/MOM6#18