Scale cluster glyphs based on number of underlying markers

Scale function is 2nd order model y = k*x^2 / (x^2 + b)
Coefficient k sets the max scale factor, i.e., y(inf) = k
Coefficient b computed to always set min to 1, i.e., y(2) = 1.0
New member MaxClusterScaleFactor sets k, default value 2.0
Replaced "MarkerCount" point data with "MarkerScale"