Changed the interface to work with sherpa2. The main changes to previous...

This change to the SherpaInterface will only work if sherpa in a version >2.0.beta2 is available in the CMSSW release, so sherpa2 from the CMSDIST package has to be included!

Changed the interface to work with sherpa2. The main changes to previous interface for sherpa2 is the change in the handling of the random number engine. The previous version was uninitialized at runtime, in order to fix this an unnamed namespace is used to store the first reference to the RNG and hand it over to the Sherpa external RNG class