ttools: CombinationConfigKey may optionally allow null value

The null value will be used to indicate 2-axis zooming for cube surface.
ttools: accommodate pointZoom as distinct from centerZoom in cube

This will allow position-sensitive 2-axis anisotropic drag-zooming
for 3d plots.  The actual 2-axis zooming code (pointZoom method
in CubeSurface) has not been written yet.
ttools: implement drag-zoom in anisotropic 3D plot

This drag-zoom mode zooms in the two dimensions corresponding to the
cube face which is most nearly facing the viewer.
ttools: fix CombinationConfigKey specifier

Although CombinationConfigKey optionally allowed null values,
its Specifier (GUI component) did not provide an interface to allow
a null.  Fixed that.