Update GeoModel-version.cmake

This MR bumps the GeoModelVersion from 5.5.0 to 5.6.0, which we use as the basis of the next release.

Main changes:

• A restructuring of classes to allow the introduction of virtual geometry, now taking the form of alienable tracking surfaces.
• Allowance for mutable materials in GeoLogVol.

cmake/GeoModel-version.cmake

 # Set up the version of GeoModel as a cache variable, so that other
 # sub-projects could use this value.
-set( GeoModel_VERSION "5.5.0" CACHE STRING
+set( GeoModel_VERSION "5.6.0" CACHE STRING
     "Version of the GeoModel project" )