shortcut for 'lhapdf-config --version' call, which is slow and responsible for timeout errors in MG

We are seeing timeout errors in madgraph, when it calls 'lhapdf-config --version'. Indeed, our (custom) lhapdf-config is sometimes quite slow as we need to look up paths on cvmfs to find lib and data directories. However, if we are only interested in the lhapdf version, we can get this information simply from $LHAPDFVER, which is obviously faster and implemented here. This is marked as urgent as the issue blocks the creation of a new AthGeneration release.