SPI-2271: Switch to HepMC3 as default HepMC library (where possible) in LCG nightly dev builds
In this merger request:
- Introduction of the HEPMC_VERSION variable to allow for switching between the default HepMC library, setting it to either 2 or 3;
- Default HepMC library switched to HepMC3;
- Building of mctester, taoula++ and photos++ against either HepMC3 or HepMC2, not both;
- Not building vbfnlo against HepMC at all if HepMC3 is set as the default.
Edited by Dmitri Konstantinov