GeoLib GeoObject retrival rewrite.

This is part of 2D ground water flow simulation changes.

It consists of a rename from getGEOObject to getGeoObject (this is touching GUI), and a rewrite of this function for better handling of missing geometries.

One thing I don't like myself (and open for _later_ discussion) is the switch-case statement on type. IMO such things should not be a part of polymorphism capable language.

@rinkk Would you please check if the rename breaks the GUI?