Feat/atmos sea ice land indicators

[AntonioCC808]

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[chpolste]

Thanks! Just the cell output and link in the docs to fix, the rest looks good! My preference would also be to go with sea_ice/sea_ice.py or ocean/sea_ice.py for that submodule.

With the amount of indicators growing, it would be good to have a discussion on the organisation and naming of submodules in ek-climate soon. The overlap with hydro brings up the question if we want to spread some functionality to the more specialised packages. The seaIce naming brings up the question if we want to mirror the xclim structure exactly (probably not). We're also 3 levels deep with submodules now (e.g., .indicators.atmos.precipitation) while the other earthkits tend to keep things quite flat. That's for another PR though...