Locating vertical coordinate file (`fv_core.res.nc`) on HPC cluster

[mrudko]

The code changes in this PR aim to facilitate setting up ML workflow on HPC cluster. Preparing the full configuration file results in retaining the path to the vertical coordinate file (fv_core.res.nc) as specified on the Google Cloud platform (e.g. gs://some/path). To perform numerical simulations, the user has to modify the path manually.

Added public API:

• c48_initial_condition_overlay function accepts vertical_coordinate_file variable (config.py); the same variable (attribute) was added to the InitialCondition class (prepare_config.py)

Refactored public API:

• FV_CORE_ASSET dictionary was moved into thec48_initial_condition_overlay function.

• [x] Tests added The test was added to test_config.py script. The test validates the path to vertical coordinate file (fv_core.rs.nc). The regression test was modified in the test_config.py script. Two tests were added to test_prepare_config.py. One test validates the default value of vertical_coordinate_file variable, while the other one ensures the safe transfer of the same variable from minimal to full configuration files.

Resolves #2197

[spencerkclark]

Integration tests passed (link here). I'll enable auto-merge to merge this once CI passes again after fixing the typo in the test name (no need to run the integration tests again since this only affects testing code).