Make the build of GeoModelTools independent.

GeoModelTools can now be built by using the -DGEOMODEL_BUILD_TOOLS=1 option when building the whole monorepo; or, it can be built individually by calling cmake over its folder (cmake ../GeoModel/GeoModelTools). For the second option, you must have the base packages (GeoModelCore and GeoModelIO) installed on your system already.

This MR also fixes the build of GeoModelExamples with the new individual builds mechanism.