Lazy loading of ERA5 observations for Google Cloud Platform

[kysolvik]

Problem: Currently to generate observations from ERA5 on GCP, we have to download all the data (for a specified window in time and space) into a Data object, and then run an observer on that object. There's no way to generate observations from the ERA5 data without downloading the fully region/time-window first.

Possible solution: By interfacing with xarray, we could sample times/locations and load only the data we need.

[kysolvik]

This has been solved with the xarray updates in dev/xarray_base. Once that's merged we can close this.

[StevePny]

@kysolvik is this resolved by #56 ?

[kysolvik]

@StevePny Yes, closing