Build mcconfigurator without GEANT3

[olantwin]

Is your feature request related to a problem? Please describe.

SHiP doesn't use GEANT3. We would like to remove it from our stack, but to move to the YAML-based config, we need to build mcconfigurator, which depends on GEANT3.

Describe the solution you'd like

Remove the dependency for mcconfigurator or make the new configuration method available without the GEANT3 dependency.

Describe alternatives you've considered

We can patch FairRoot ourselves, but we'd like to stay with the upstream version if possible to avoid diverging.

Additional context