Implement map + chart for the 4 currently available CMIP6 indicator ARDAC items

[cstephen]

This PR consists of the maps + charts of the following CMIP6 variables available through our development API:

• Freeze/Thaw Cycle
• Summer Days
• Deep Winter Days
• Maximum 1-day Precipitation

To test, run a local instance of the Data API from the cmip6_indicators branch, pointing at Zeus:

cd data-api
git checkout cmip6_indicators
git pull --ff-only
export FLASK_APP=application.py
export API_RAS_BASE_URL=https://zeus.snap.uaf.edu/rasdaman/
pipenv run flask run

Then run this branch of the ARDAC Explorer against the local API, and against Zeus for its Rasdaman server:

cd ardac-explorer
git checkout implement_cmip6_items
export SNAP_API_URL=http://localhost:5000
export RASDAMAN_URL=https://zeus.snap.uaf.edu/rasdaman/ows
npm run dev

Then view the available CMIP6 items here:

http://localhost:3000/item/indicator-ftc-cmip6 http://localhost:3000/item/indicator-su-cmip6 http://localhost:3000/item/indicator-dw-cmip6 http://localhost:3000/item/indicator-rx1day-cmip6