C_G-CK C_G comet fixed frame (CK frame)

 A body-fixed frame is defined using standard 
 body-fixed, CK-based frame formation rules: 

  -  +Z axis is toward the North pole;

  -  +X axis is toward the prime meridian;

  -  +Y axis completes the right hand frame;

  -  the origin of this frame is at the center of the body.

The orientation of this frame is computed by evaluating
corresponding C_G comet attitudes kernels.