Tests are very slow due to big file downloads

[danielolsen]

On a pretty good home internet connection, with nothing else going on:

• prereise/gather/demanddata/nrel_efs/tests/test_get_efs_data.py::test_download_flexibility_data takes >2 minutes
• prereise/gather/demanddata/nrel_efs/tests/test_get_efs_data.py::test_download_demand_data takes >1 minute
• no other tests take more than 10 seconds

Can we cut this time down somehow, perhaps by testing the download functionality on smaller files?

[lanesmith]

Yeah, the files are pretty large so downloading and extracting takes a while. I could write tests for the private functions (e.g., _download_data, _extract_data) instead. This should allow us to test the functionality on smaller files.