#18 Enhance Parametric Applet

[Tom Downey]

Some enhancements to the parametric applet:

- add a slider option, plus direct k input, so that the
user can control the tracing
- add an option for a tangent line at the trace point,
and the option to show the slope at that point (if
there is a slope)
- add a way to show implicit functions; maybe just by
naming the example with the equation of the implicit
function, then giving the parametric version of that
function
- option to graph the derivative of a parametric
function (or maybe a new ParametricDerivatives applet...)

Ideally, I'd love a tool for graphing implicit
functions directly...