File download for testing structured mesh case

[rajeeja]

FAILED test/io/test_structured.py::test_read_structured_grid_from_ds FAILED test/io/test_structured.py::test_read_structured_grid_from_latlon

Both of these tests failed on a few occasions because they tried to download a test data file (air_temperature.nc) from a GitHub URL. The connection to github.com timed out, which caused the tests to crash.

requests.exceptions.ConnectTimeout: HTTPSConnectionPool(host='github.com', port=443): Max retries exceeded...