Add warning if we detect OSCAR is outdated?

[fingolfin]

Sometimes for various reasons, people end up with outdated OSCAR versions, even after seemingly installing OSCAR for the first time. A typical reason is that some other package is in their primary environment which is incompatible with the current OSCAR version, and hence an older OSCAR version gets installed. Which then leads to all kind of confusion.

Perhaps we could add some code to OSCAR which checks the current package registry (the one already on the users disk, I am not suggesting network access here) to see if there is a newer OSCAR in it; and if so, issue a warning. It could provide more information about the problem and how to fix it (possibly also a link to a website with more info). So e.g.

This is OSCAR 0.12.3, which is outdated, OSCAR 1.0.3 is current. We recommend you update it. To do so, start a fresh Julia session and enter using Pkg; Pkg.update(). To check whether this worked, enter using Pkg; Pkg.status("Oscar"). If OSCAR is still outdated, some other package in your environment may prevent the upgrade. You could try to remove some or all of them, then try again to update OSCAR.

Of course this won't help immediately, as old version won't have that check, but if we add it now, we would benefit from it in the years to come. (We could also add new releases to some of the older 0.x.y release series and/or ask for certain old versions to be yanked from the registry, but I don't think this is really necessary.

A drawback of this approach is that if someone deliberately wants an older OSCAR version, the warning might be annoying. I guess we could add a way to turn it off (via some environment variable?).