Switch default IERS to use a supported CDDIS access mechanism #9963

Continuing the never-ending saga of IERS file availability... I got the following notice as part of a question to CDDIS (the IERS mirror server) today:

FTP access is being phased out over the next few months. CDDIS is STRONGLY recommending all users begin to transition to use https (https://cddis.nasa.gov/archive) as the preferred method of access to the CDDIS archive. Download procedures can be found at: https://cddis.nasa.gov/About/CDDIS_File_Download_Documentation.html, along with a Frequently Asked Questions (FAQ) link and links of how to use cURL, wget and Python (https://cddis.nasa.gov/Data_and_Derived_Products/CDDIS_Archive_Access.html)

CDDIS has also made available ftp-ssl (gdc.cddis.eosdis.nasa.gov) as a secondary method to access the archive. In some cases the use of ftp-ssl may make the conversion of existing scripts easier than using https. Examples of how to use ftp-ssl are listed at: https://cddis.nasa.gov/Data_and_Derived_Products/CDDIS_Archive_Access.html.

This might be ignorable if #9453 can be fixed in time but we probably shouldn't count on it...

I looked a bit at both of these options - the https solution won't work because it requires a username and password. ftp-ssl seems likely to be the better bet, as it allows anonymous access. I'm going to try to whip up a PR to get this working, but figured I'd better log the issue first in case my PR is not successful.

cc @pllim @mhvk