Only Persefone 0.5.0 works at the moment, so adjust compat section accordingly

With Persefone 0.5.1, there is an endless stream of

reached the end of a @resumable function

printed to the screen at 100% CPU usage.

I think this is caused by a change in Persefone, where the landcover
etc files are now found relative to a mapdirectory.  This setting is
not present in the GUI, printing an error to the screen but then
continuing.

It is not sufficient to just change guiparameters.toml, as parts of
the GUI code also expect to access files at a certain location, and
these parts of the code must also be changed.