CMake Cleanup, master branch (2019.10.30.)

See merge request !1

GeoModelWrite was not using this module anyway. (It does not depend on
Eigen directly.)

In exactly the same way as for GeoModelCore. It should be fine-tuned
later on...

Now that the base project uses the "GeoModelCore::" namespace on its
exported targets...

Fixed the name of the exported version file, so that CMake would
actually take it into account.

Also updated the main project description file to ask for an explicit
(minimum) version of GeoModelCore, and fixed the name of a variable
in the file.

Made the same sort of updates as in GeoModelCore. Though things had to be
done in an even more complicated way in this project.