add main and have CSVConvert as CLI script

[mshadbolt]

This change adds CSVConvert as a project script which allows a user to simply use the name of the script to call the script, rather than the whole path. This required adding a main method to the script but hasn't changed any functionality. Expected functionality is now

cd clinical_ETL_code
pip install -e .
CSVConvert --input <path to raw_data> --manifest <path to manifest>

[daisieh]

LGTM! Works as expected.