Deprecation warnings

[nocollier]

As part of the ESGF2-US review, a reviewer pointed out how difficult a transition it will be for many users when we phase out the use of wget scripts. Once we have dates official dates approved, we should introduce deprecation warnings when the scripts are run. I envision this as simple text that we would echo:

• Perhaps at the beginning of the script and the end, just in case users didn't see it at first
• The text should give a clear date for end of life
• The text should point to a blog post / page which explains the reasons for the transition and explains how alternatives work