Fix download test data script.

[mvdebolskiy]

Just updated the links in the driver/download_test_data.sh and README.md to be compatible with the github workflow. Running

./cosp2_test  cosp2_input_nl.um_global.txt
cd ../
python compare_to_kgo.py data/outputs/UKMO/cosp2_output.um_global.gfortran.kgo.v002.nc data/outputs/UKMO/cosp2_outpu
t.um_global.nc --atol=0.0 --rtol=0.000

Does not give an error.

Added KGO_VERSION env variable `driver/download_test_data.sh similar to github workflow.
Ideally, users should be able to check out multiple kgo versions with the download script. Can remake it as an argument to the script.

Also, added .gitignore to ignore all the files created during driver build and download.

Issue related: #83

[alejandrobodas]

@mvdebolskiy thanks for proposing these changes. I've fixed the CI tests, please can you merge the recent changes into your branch?

[alejandrobodas]

Thanks, it looks good to me. @dustinswales , would you mind reviewing this PR? If you are happy with it, then please go ahead and merge.

[alejandrobodas]

@mvdebolskiy thanks for your contribution!