Support arbitrary or admin-provided regions

[nuest]

From tomniers/geoOJS#14

Instead of administrative units, also other spatial entities can be of interest. For an ecological journal, World Protected Areas (protectedplanet.net/en) are probably more interesting, than administrative units. So the integration of alternative spatial entities would be worthwhile. Or maybe not only the initial integration but also the possibility for the journal operators to integrate alternative spatial entities themselves, e.g. based on a GeoPackage.