Divergence-Curl Demo

This demo allows the user to input an arbitrary 2-D vector field as a function of Cartesian (x,y) and/or polar coordinates (r,t). The value of the vector field is shown with arrows using the QUIVER command. The user can overlay the divergence, curl, and magnitude as scalar fields shown in color. This serves as a simple, easy-to-use visualization tool for vector fields, and because the user can input arbitrary functions, allows for more interactive experimentation than simply looking at a few graphs out of a textbook.