Allow containers in decay treatment

[Frank Siegert]

It would be nice, if it was possible to use containers in the decay treatment. Example use case:

(run){
  PARTICLE_CONTAINER 9923[m:-1] Zgamma 23 22
}(run)

(processes){
  Process 93 93 -> 9923[a] 24[b] 93{2}
  Decay 9923[a] -> 11 -11
  Decay 24 -> -11 12
  [...]
  End process
}(processes)

Even if only Comix can use that it would already be quite useful.

[Stefan Hoeche]

Implemented on trunk r18762. Note that the solution to ticket #203 requires that particle containers have a unique mass, which is normally set by their constituents. If a Z/y-container is constructed, this mechanism must be disabled by setting the mass to -1 explicitly

  PARTICLE_CONTAINER 9923[m:-1] Zgamma 23 22