Add support for `SPHERE_SWEEP`

[wanglongqi]

Does add SPHERE_SWEEP hard to do? I am a little confuse on how to add this one?

SPHERE_SWEEP:
  sphere_sweep {
    linear_spline | b_spline | cubic_spline
    NUM_OF_SPHERES,

    CENTER, RADIUS,
    CENTER, RADIUS,
    ...
    CENTER, RADIUS
    [tolerance DEPTH_TOLERANCE]
    [OBJECT_MODIFIERS]
    }