The converter used to convert IterativeEquation to a specific PennerEquation
defined in the PennerEquations class based on a string. This comes in really handy when
we are creating the "IterativeDoubleAnimation" in markup. All we need to do is set the "Equation" property
on the IterativeDoubleAnimation to a string corresponding to a PennerEquation from the PennerEquations class.
This class contains a rich set of pre-defined PennerEquations and their implementations.
Any of the PennerEquations can be used as the equation for an IterativeEquation.