ENH: cache downloaded atmosphere netCDF datasets (#654) - #1137

Not ready to merge yet. Three items, the third being the one I care most about.

1. Three of this PR's own new tests fail on all six platforms.

FAILED tests/unit/environment/test_environment.py::test_set_atmospheric_model_normalizes_shortcut_case_for_forecast
FAILED tests/unit/environment/test_environment.py::test_forecast_shortcut_and_dictionary_are_case_insensitive[AIGFS]
FAILED tests/unit/environment/test_environment.py::test_forecast_shortcut_and_dictionary_are_case_insensitive[HRRR]

IndexError: too many indices for array: array is 0-dimensional, but 2 were indexed
  rocketpy/environment/environment.py:1637

2. pylint fails (exit 8):

rocketpy/environment/atmosphere_cache.py:158:0: R0915: Too many statements (30/25)
rocketpy/environment/atmosphere_cache.py:270:0: R0915: Too many statements (35/25)
rocketpy/environment/environment.py:1237:4: R0912: Too many branches (28/25)

Please split those into smaller functions rather than adding # pylint: disable.

3. Cache invalidation — the design point I want resolved before this lands.

The cache is enabled by default, and the key contains neither a TTL nor the forecast model run/cycle. So a user who runs a GFS forecast today and again tomorrow silently gets yesterday's forecast, with no indication anything is stale. For forecast products that is a wrong-answer bug, not a stale-file annoyance, and it is the kind of thing that is very hard for a user to notice.

What I would want:

• the model run / cycle timestamp as part of the cache key, or a TTL, so a new cycle can never be served from an old file;
• caching off by default until we have confidence in the invalidation;
• and it is worth distinguishing product classes: reanalysis/archive data (ERA5 and friends) is immutable and can be cached indefinitely, forecasts cannot.

Happy to revisit once those are addressed.