[crossfire] moving gps code

[Nicolas Weeger]

Hello.

A while ago i added a "ground positionning system", item that lets you
know your location in the world.
AFAIK you can only find it on mlab maps, it's not yet on other maps.

Unless someone objects with good arguments, i'll move the relevant code
to a Python script. Since it's not a mandatory behaviour (it's more a
convenience), a script is the perfect way for it, and that means i can
trash the specially introduced item type.

Note that there shouldn't be any conversion issue between old & new
implementation as long as archetypes and maps are updated at the same time.

Ryo