Added working test_cli.py using valid string inputs

[agstephens]

@alaniwi: This single test module should be enough to guide you in how to set up the CLI interface. A few notes:

Run with:

pytest -c null tests/test_cli.py 

Note that this uses pytest.mark.parametrize which allows you to define a list of inputs that get parametrised into a single test - so that it runs it once for each input. Note that the inputs demonstrate that we can send strings in for each argument, e.g.:

[CMIP6_ID, "1900-01-16/1901-12-16", "month:jan,apr,aug|year:1901", "0,-10,120,40", "100000,85000"]

[agstephens]

@alaniwi: the CI is not passing on Python3.7 at the moment. Please ignore that as we are no longer building daops for py3.7.

[alaniwi]

Thanks. Have had a look. Merging into "cli" for now.