Lightsource shade method parameters for color range definition

Added vmin and vmax arguments to LigthSource shade method which, if set, changes the lower and upper border of the colorspace. Compare with method contourf where these parameters are available. Basically they are needed if one has data in a smaller range than the colospace one wants to use. Normally the shade method just takes data.min() and data.max() as the range, by defining vmin and vmax thos get overwritten.