Switch to common/public installation of LHAPDF

[bistapf]

As an intermediate working version with PR#218, the script to generate LHE on the fly with Madgraph, https://github.com/HEP-FCC/EventProducer/blob/master/bin/submitMG.sh, uses a custom installation of LHAPDF. This should be either moved to a public groupspace directory, or a common LHAPDF installation (ideally from the key4hep stack) should be used.

Complication is that legacy files require support of old MG & LHAPDF versions.