PR mirror is never refreshed

[gouttegd]

The way the Makefile for this PR slim works, once a local copy of PR has been downloaded into the mirror directory, it will never be refreshed for as long as that copy exists, and the slim may then be re-generated using a source PR that could be several months old.

Since several people can (and do) update the slim, this can lead to the slim sometimes going “backwards“, because of a scenario like the following:

• Alice updates the slim (by running make all on a freshly cloned repository) on January 1st; she now has a local copy of PR from that day.
• Bob updates (also by running make all on a freshly cloned repository) the slim on March 1st; he now has a local copy of PR from that day.
• On April 1st Alice updates the slim again, from the same repo as before. Her local copy of PR is not refreshed (since she already has a mirror/pr.owl file, which is enough to meet the requirements stated in the Makefile), so her updates is based on the version of PR dated from January 1st.
• On June 1st Bob updates the slim again, from the same repo as before. Again, his local copy is not updated, so his update is based on the version of PR dated from March 1st.

All subsequent updates made by Alice and Bob will go back and forth between the version of PR from January 1st (first and only version downloaded by Alice) and the version of PR from March 1st (first and only version ever downloaded by Bob), forever until they either work from a newly cloned repository or they manually delete the mirror/pr.owl file to force downloading a newer version.

This is not a theoretical scenario, as it is exactly what happened over the last few days, causing an issue that had been fixed months ago to suddenly reappear in Uberon (obophenotype/uberon#3461).

There may be some value in not re-downloading PR all over again every time someone runs make all, but at the very least the README needs to be updated to remind people to delete the mirrored files from time to time -- especially if the previous update was made by a different user, who may have had a more recent version of PR in their mirror directory.

Ideally the Makefile should have a clean target to automatically wipe out the mirror, so that refreshing could be done in one step with make clean all.