Added a simplified interface to the type information system in
ThePEG. For simle classes there is no need to specialize the
ClassTraits and BaseClassTrait classes and to have a static member
variable of ClassDescription in the class (as in the full ThePEG type
info system). Instead it is enough to have one statically initialized
variable of one of the DescraibeClass classes for each class. The
Abstract and NoPIO versions of this class should be used for abstract
classes and classes without persistent I/O functions respectively.