Compare moodle `timemodified` with timestamp of local files

[septatrix]

Sometimes course instructors upload newer versions of a file to change some mistakes etc. This would be a relatively simple¹ way to catch those cases and download the newer version.

However if this is implemented we would also provide a solution for cases where all timestamps get updated (e.g. copying the files from one pc to another). For linux this could be done by a recursive touch to a day in the past however it would also be possible to use os.utime to set the timestamp to the one in Moodle. This could actually be done for every downloaded file

¹ In relation to always downloading every file and comparing the checksum.