New treatment of the beam type in the AliGRPObject (with corresponding changes in...

New treatment of the beam type in the AliGRPObject (with corresponding changes in AliGRPPreprocessor):

AliGRPObject::GetBeamType() --> returns "A-A", "p-p", "p-A", as needed for the reconstruction
ALiGRPObject::GetBeamTypeFromLHC --> returns string containing EXACTLY what is published by LHC (e.g. "PB82-PB82")
AliGRPObject::GetSingleBeamType(Int_t i) --> returns the beam type for each beam, defined as a string with the convention: beam type = A*1000+Z