Master geomodel new monorepo

This MR introduces the External package to build the new GeoModel 'monorepo' (1). It also introduces a new FindGeomodel.cmake module to find the GeoModel libraries and headers correctly.

The MR also removes the earlier GeoModel single-repositories (GeoModelCore, GeoModelIO, GeoModelTools).

This MR supersedes !703 (closed).

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The CMakeLists.txt files of the Athena packages using GeoModel packages are expected to be updated in the following way:

• find_package(GeoModelCore) —> find_package(GeoModel)
• ${GEOMODELCORE_LIBRARIES} —> ${GEOMODEL_LIBRARIES}
• ${GEOMODELCORE_INCLUDE_DIRS} —> ${GEOMODEL_INCLUDE_DIRS}

A related MR is being pushed to update the Athena packages.

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(1): https://gitlab.cern.ch/GeoModelDev/GeoModel

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Cc: @akraszna , @tsulaia , @boudreau , @dellacqu , @mbandier , @dcasper