ERA5 initial conditions for CliMA simulations

[nefrathenrici]

New package: InitialConditions.jl

• Adds a package that downloads, processes, caches, and validates the ERA5 initial conditions that CliMA simulations start from.
• One entry point does everything: ERA5.fetch_initial_conditions(date) returns the directory holding the six output files for that date. Outputs one file each for the atmosphere state, SST, sea ice, integrated land, bucket land, and albedo, read by ClimaAtmos, ClimaLand, and the prescribed ocean and sea ice models.
• Processing follows the WeatherQuest pipeline that made the wxquest_initial_conditions artifact, with one change: the atmosphere state uses the 37 ERA5 pressure levels instead of the 137 model levels.
• Processing functions are usable on their own. They read by variable name, not file layout, so a caller who already has ERA5 downloads can skip the network.
• Downloaded files go to a Scratch.jl directory and stay there across runs. INITIAL_CONDITIONS_CACHE_DIR canm be used to specify a shared cluster cache.
• Default test suite replaces CDSAPI.retrieve with a fake that writes small fixtures. No network, runs in seconds. A separate network test exercises the real CDS path, behind ERA5_NETWORK_TESTS=true.