This should complete all of Joyce's options for constructing
a point by being perpendicular to something. Also fixes coloring
of the sphere and adds the planeSlider method for constructing a
point.
Working toward successfully rendering Book Eleven. Specific changes in
this commit include:
* Improvement to coordinate system/initial view for 3D constructions
* More attention to ambient plane for some 2D commands
* Add sphereSlider construction method for point
* Add intersection construction method for circle (intersection of
two spheres)
* Fix display/non-display of perimeter of sectors
* Add sphere construction by radius
* Add polyhedron construction methods parallelepiped and pyramid