Put the extensions in a new git repository. Use the git repository as a submodule

Move ATLAS-Extensions from GeoModel Proper into a submodule, now called ATLASExtensions (without the hyphen). This is part of Marilena's refactoring proposal.

ATLAS-Extensions/ATLASMagneticFieldMapPlugin/cmake/BuildType.cmake

-
-# Author: Marcus D. Hanwell
-# Source: https://blog.kitware.com/cmake-and-the-default-build-type/
-
-# Set a default build type if none was specified
-set(default_build_type "Release")
-
-# TODO: at the moment, we want to build in Release mode by default,
-# even if we build from a Git clone, because that is the default mode
-# for our users to get the source code.
-# But maybe we will want to change this behavior, later?
-# if(EXISTS "${CMAKE_SOURCE_DIR}/.git")
-#   set(default_build_type "Debug")
-# endif()
-
-if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
-  if( COLOR_DEFS )
-    message(STATUS "${Blue}INFO: Setting build type to '${default_build_type}' as none was specified.${ColourReset}")
-  else()
-    message(STATUS "INFO: Setting build type to '${default_build_type}' as none was specified.")
-  endif()
-  set(CMAKE_BUILD_TYPE "${default_build_type}" CACHE
-      STRING "Choose the type of build." FORCE)
-  # Set the possible values of build type for cmake-gui
-  set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS
-    "Debug" "Release" "MinSizeRel" "RelWithDebInfo")
-endif()
-