Changed the parent prototype class for GeomXspline

[mtorchiano]

In order to get a similar result as the base graphics xspline function, the base proto class has to be a path and not a line.

In most cases there will be no difference, although in all the cases when the order of the control points has not an increasing x variable the result with a line will be a jittered output (see bug report #64 for details)

Fixes issue #64

[mtorchiano]

The failure is not related to the proposed change there is a general problem in setting up the VM