to make it selectable via 'pytest -k output_data_format'

1. Date information is moved onto a column.
2. Test column exists and its type at once.

1. Date information is in a column instead of index.
2. UVI is now float instead of int.

…r loop

Note to self: 'pytest --disable-recording' will perform tests **without** VCR functionality,
so you can run it to see if the tests pass and it's the time to re-record the cassettes to
match up with the now-successful tests.

fix: Improve file saving and its corresponding tests

fix: Improve `get_city_forecast` to pass tests

test: Mark slow tests