Add intermediate remote data access tutorial

[dcherian]

Illustrate

• opendap / thredds
• datasets on S3
• datasets using gcsfs
• fsspec

Example code:

import gcsfs

fs = gcsfs.GCSFileSystem(token="anon")
ds = xr.open_zarr(
    fs.get_mapper("gs://pangeo-noaa-ncei/noaa.ersst.v5.zarr"), consolidated=True
).load()
ds