Check s3 bucket for cached era5 downloads before downloading

[rileyhales]

If there is era5 data staged on the s3 bucket and the downloader machine turns on, the code fails to find things on its own local disc and tries to download. It does not check to see if those files are already successfully uploaded to the s3 bucket when it could safely skip and trigger computations.

-> check the s3 bucket *and the local disc for staged era5 files before triggering more downloads.