Physics decays not defined

[Jochem Snuverink]

Many tests (see cdash) are failing with messages like:

G4VDecayChannel::FillDaughters  [ kaon- ]1:mu- is not defined !!

The BR of this decay mode is set to zero 

[Laurie Nevay]

Contrary to the geant4 documentation, the required (lepton) particles aren't constructed by default. This has now been fixed by constructing EM physics as well.

In future, this could actually be changed to remove the EM physics and purely switch to only the lepton construction - but would require testing. For now, works ok.