Introduced new GeoModel external for the standalone GeoModel kernel library

@akraszna , could you please take a look at my changes and let me know what needs to be fixed?

I'm already aware of one issue: by whatever reason GeoModel libraries are built and installed into the ...path.../lib64 directory instead of ...path.../lib (where the rest of external libraries are installed). I could not figure out why this happens... Any advise?

... I'm sure you will have other comments for my changes too. Thanks in advance :)

Tagging @wlampl @emoyse @asalzbur such that our software coordination is aware of this development