x = InitialXExtP(xp)
Initial values for extended power function fit.
If argument is passed, it is assumed to be the
parameters for best ordinary power function fit.
8/7/00 dhb Modify for new equation.
11/16/06 dhb Order of vector was reversed relative to how it’s used in
FitGammaExpP. Make default guess for gamma 2.
11/16/06 dhb Adjust initial offset for [0,1] world.