This MR removes the now obsolete GeoModelCppHelpers package: all its methods have been moved to the global helper package GeoModelCore/GeoModelHelpers.
This "fixes" the issue with the CMake include configuration spotted by @akraszna in !333 (closed) by completely removing the offending package.
The MR also fix one of the GeoStrUtils helpers by adding a missing template type (Cc: @jojungge)