handling of off-shell particles

due to the miss match of particle masses between external models/modules and corsika, particles appear to be off-shell in corsika. One example is \rho mesons in Sibyll.
Important points are

• need to know when this occurs (bookkeeping option?)
• in mild cases (how to define?) the particle masses can be synchronized by adjusting the energy of the particle (slightly violating energy-momentum conservation)
• in extreme cases we want to throw an error as these may indicate a problem in the model/module or the interface to corsika.
• some form of validation that the shifting does not change the model output too much.