Close GitHub Discussion forums on Pythia repos

[clyne]

Now that we're directing discussions to the Pangeo Forum per #289, we should consider closing the GitHub Discussion forums open on the various GitHub repos. Having multiple discussion forums causes confusion for the community and places an additional support burden on the Pythia maintainers. I don't think this is a high priority issue, but if people continue to create new discussion threads on the Pythia repos we might want to reevaluate.

[brian-rose]

@dcamron will investigate the possibility of closing the discussion boards to new posts but retaining the existing discussion threads.