Acceptance tests failing outside of environment a due missing packages

[benowen-bom]

Some acceptance tests are failing due to missing packages. Issue is that pygam, pysteps and python-stratify are missing from certain environments, namely: environment_b.yml and conda-forge.yml.

The associated acceptance tests should look for these packages and skip if missing (as done in improver_tests/acceptance/test_apply_rainforests_calibration.py).

Related issues: #2341.

Further details:

Running the acceptance tests on recent build of environment_b.yml (using changes in #2344) results in the following test failures.

=========================================================================== short test summary info ============================================================================
FAILED improver_tests/acceptance/test_apply_samos_coefficients.py::test_normal_gam_coordinates - ValueError: Failed to load /home/548/bo1606/improver_suite/improver_test_data/data/apply-samos-coefficients/gam_configs/samos_gam.pkl: No module named 'pygam'
FAILED improver_tests/acceptance/test_apply_samos_coefficients.py::test_normal_gam_cubes - ValueError: Failed to load /home/548/bo1606/improver_suite/improver_test_data/data/apply-samos-coefficients/gam_configs/samos_gam_additional_features.pkl: No module named ...
FAILED improver_tests/acceptance/test_apply_samos_coefficients.py::test_normal_sites - ValueError: Failed to load /home/548/bo1606/improver_suite/improver_test_data/data/apply-samos-coefficients/gam_configs/samos_gam_sites.pkl: No module named 'pygam'
FAILED improver_tests/acceptance/test_apply_samos_coefficients.py::test_no_coefficients - Failed: DID NOT WARN. No warnings of type (<class 'UserWarning'>,) were emitted.
FAILED improver_tests/acceptance/test_stochastic_noise.py::test_basic - ModuleNotFoundError: No module named 'pysteps'
FAILED improver_tests/acceptance/test_stochastic_noise.py::test_scale_non_positive_noise - ModuleNotFoundError: No module named 'pysteps'
============================================= 6 failed, 503 passed, 23 skipped, 7918 deselected, 308 warnings in 408.67s (0:06:48) =============================================

Acceptance criteria:

• Acceptance tests no longer fail due to missing packages for environment_b.yml
• Acceptance tests no longer fail due to missing packages for conda-forge.yml