Adaptive plotting for parametric plots

I've just checked in support for adaptive plotting for parametric plots.
I've currently made it the default, but if there are issues, you can set
use_adaptive_parametric_plot to false to get the original method.  If
adaptive plotting works out well, the variable use_adaptive_parametric_plot
will be removed.

For an example, compare:

use_adaptive_parametric_plot:false;
plot2d([parametric, fresnel_c(t), fresnel_s(t), [t,0,100],[nticks, 1000]]);

versus:

use_adaptive_parametric_plot:true;
plot2d([parametric, fresnel_c(t), fresnel_s(t),[t,0,100]]);

Hope this is useful,

Ray