Add logic for remote or local files in NetCDFSource

[scottyhq]

This PR follows up #82 to do a few things

• add logic to call either fsspec.open_local() or fsspec.open() for NetCDFSource
• makes chunks kwarg optional for RasterIOSource
• adds many more network/remote loading tests for NetCDFSource and RasterIOSource including a mock S3 server

Fixes:

• https://github.com/intake/intake-xarray/issues/88#issuecomment-730798001
• https://github.com/intake/filesystem_spec/issues/476#issuecomment-731831365

cc @rabernat @martindurant

[scottyhq]

quick update - I added quite a few new tests under test_remote.py and also added test_network.py which includes simple examples from linked issues reading over the network from http, s3, gcs (these currently are not run via CI though).

added a py39 environment file and test, and now install xarray master for upstream CI test.

I realized in putting a few more tests together that open_rasterio() required chunks as a positional argument, so I set that to a default of chunks=None since it's sometimes nice to bypass dask. This is checked in test_remote::test_open_rasterio.

[martindurant]

Looks good!

For s3, you could use the same moto fixture that s3fs does, rather than hitting the real backend. I would not try to implement the gcsfs fixture here.

[scottyhq]

For s3, you could use the same moto fixture that s3fs does, rather than hitting the real backend.

Woohoo! That took some figuring out, but I've added basic tests for rasterio and netcdf with moto.

If it's okay with you @martindurant I'll go ahead and merge?