Updated the minimum CMake version to reflect the recent changes (down to 3.10,...

Updated the minimum CMake version to reflect the recent changes (down to 3.10, because of Ubuntu 18)

CMakeLists.txt

 # Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 
 # === Preamble ===
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.10)
 project( "GeoModel" VERSION 1.0.0 LANGUAGES CXX )
 # enable_testing()