Earth orientation updates

Edit the file oyster/usno/cronjob to contain the proper path to the update shell script, and then use the command

crontab cronjob

to install a job which fetches every Thursday the new UT1 and polar motion data from a computer at the US Naval Observatory. Otherwise, the database usno/mark3.dat obtained with the download will not be updated and eventually will not provide Earth orientation data for the date observed. In this case, a warning message will be displayed, which, however, can usually be ignored in all cases but astrometry.