Add error when output_dir not specified

[colin-harkins]

This pull request raises an exception when output_dir is not specified in the control .yaml file.

[zmoon]

Not sure what is causing the failure. The message says "NetCDF: Filter error: bad id or parameters or duplicate filter", but it was passing before and the versions of netcdf and xarray are the same.

Ah but netcdf-c (libnetcdf in the env) has changed from 4.8.1 to 4.9.1, could be Unidata/netcdf4-python#1205

[colin-harkins]

It looks like it's failing with this message:

FAILED melodies_monet/tests/test_get_data_cli.py::test_get_airnow_comp - subprocess.CalledProcessError: Command '['melodies-monet', 'get-airnow', '-s', '2019-09-01', '-e', '2019-09-02', '--dst', '/tmp/pytest-of-runner/pytest-0/test_get_airnow_comp0', '-o', 'x.nc', '--compress']' returned non-zero exit status 1.

If I understand correctly, test_get_airnow_comp calls melodies_monet/_cli.py::get_airnow, which calls monetio's airnow.add_data function to download a small amount of airnow data from the web. I'm not sure if this is the case but if that airnow data hosted on the web has changed (in a way that would raise a netCDF error when saving to that format) or is unavailable for some reason, then that could be causing this to fail without any change on our end.