3D data space to figure space conversion

Just like ds2nfu, but works with 3D plots

[Xf, Yf] = ds3nfu(HAX, X, Y, Z) ;
Takes X,Y, and Z coordinates on the axis HAX, and gives you the
X and Y position in the figure window, from 0 to 1
same idea as ds2nfu.m from file exchange, except it works with 3d input

Also includes test function to demonstrate utility
-Jeff Marks