A new physics process added to the enumeration in the Dataset class

Wrote description for other processes, whose naming was not obvious.
Short-cuts added to GenParticle

They allow to access the first mother of a GenParticle in a convenient way. Note that there is only one mother in the majority of cases.
Added access to jet charge
Bugfix: JP value of a jet could be used uninitialised

Also changed to default values of b-discriminators in Jet constructors to (-infinity).