A few changes in orekit-data convenience archive

As explained in the quick recommended setup documentation, the Orekit team provides a convenience orekit-data archive that can be used as an initial setup for new users. This is maintained as the orekit-data gitlab repository and the documentation refers to the GitLab URL that generates an archive on the fly for the master branch of the project.

The archive includes data needed for classical uses (EOP, leap seconds, ephemerides…) and an update shell script that can be used to automatically update the content by downloading files from the reference entities that publish volatile data like EOP or space weather.

Two important changes have been made on this archive recently.

The first one occurred in early April 2021 as we replaced DE-430 ephemerides by the more recent DE-440 ephemerides.

The second one occurred today (early July 2021) as we replaced USNO tai-utc.dat file by IERS UTC-TAI.history file. This move was done because USNO maia server has been decommissioned and it seems the USNO site modernization is months late. By default, Orekit is able to parse both files so this should probably be transparent for most users. Beware however that if you set up your own update scripts and download the archive (which is the regular way) rather than using git pull (which is the way Orekit repository maintainers use), then you may end up with both tai-utc.dat and UTC-TAI.history file, and Orekit will pick one or the other. This could be a problem when next leap second is introduced (no leap second has been announced yet) and only one of the files is updated. It would therefore be safer to make sure you have only one of these two files and you keep it up to date.