Improved trail behavior

[Bruce Sherwood]

By default, when you attached a curve-type trail to a moving object the radius of the curve was set to a fraction of the object's size. This had the bad effect of displaying nothing if the object was smaller than a few pixels. Now the default curve radius is 0, which means that a thin curve will be displayed even if the object itself is very small on the screen. This improvement was made in response to a user having difficulty importing an old VPython program that had very small objects and as a result had invisible trails.