Initial changes for LCG_101x with DD4hep compiler with Geant4 units
This MR adds the toolchain for LCG release 101x, based on LCG 101 but with DD4hep compiled with Geant4 units for gcc11 and clang12.
As we cannot modify LCG 101 as it has been used in released software, we created a release with identical metadata except for the DD4hep version, called 101x.
N.B. As EP-SFT will use 101a, b, c... for small updates on top of LCG 101, we used 101x.