Add six new CMIP6 data x-ray items

[cstephen]

Closes #145. Xref: https://github.com/ua-snap/data-api/pull/472

This PR implements six new "data x-ray" items based on the new CMIP6 monthly coverage on Apollo.

To test, run the data-api off the cmip6_monthly branch, pointed at Apollo:

cd data-api
git checkout cmip6_monthly
export FLASK_APP=application.py
export API_RAS_BASE_URL=https://apollo.snap.uaf.edu/rasdaman/
pipenv run flask run

Then run ARDAC Explorer, pointed at your local API and Apollo:

cd ardac-explorer
git checkout cmip6_monthly
export SNAP_API_URL=http://localhost:5000
export RASDAMAN_URL=https://apollo.snap.uaf.edu/rasdaman/ows
npm run dev

Then, test drive the maps & charts on the following items:

http://localhost:3000/item/temperature-cmip6 http://localhost:3000/item/wind-cmip6 http://localhost:3000/item/oceanography-cmip6 http://localhost:3000/item/precipitation-cmip6 http://localhost:3000/item/evaporation-cmip6 http://localhost:3000/item/solar-radiation-cloud-cover-cmip6