The PlainNumberer class is a DOF_Numberer. An object of this
class assigns the equation numbers to thee DOF_Groups based on the
order in which they are obtained from the DOF_GroupIter object
obtained from the AnalyisModel. The class is useful for situations
where the SystemOfEqn or Solver objects will impose a renumbering on
the equations, which makes performing a complex numbering of the
degrees-of-freedom a waste of computational effort.