Have NEON data for testing pre-downloaded

For our standard testing we need to have a location for the NEON data on both Derecho and Izumi, so that tests won't arbitrarily fail. This is something that @adrifoster talked about a long time ago, and we keep bringing it up as it would be good to do. But, unfortunately not high enough priority for us to spend time on.

Definition of done:
- [ ] Figure out how to do this, may require updates in CIME
- [ ] Add an option for NEON data to look for data in an optional location external to the case, and download it there as normal
- [ ] Change our test suite tests to use the downloaded data location, run the tests to populate it
- [ ] Add a separate test that ensures that we can download latest NEON data and the (this could be either a python system test or a System testlist or added to ctsm_sci tests)