Replaced GeoModelKernelUnits with Gaudi::Units
During the initial migration of Athena code to the standalone GeoModelKernel library, all usages of CLHEP units were replaced with GeoModelKernelUnits. Later I discovered that we actually have a recommendation in the ATLAS coding guidelines document that suggests the usage of Gaudi::Units. This commit implements the recommendation.
Edited by Vakhtang Tsulaia