Allow clip command to specify positions relative to center of rotation

[olibclarke@…]

Would like the "fromCenter true" option that Chimera has for setting clip plane positions relative to the center of rotation. Currently the ChimeraX clip command sets the clip plane positions relative to the center of the bounding box of what is displayed. Using the center of rotation makes more sense when the user is actively controlling the center of rotation (center of view, or click atom to set pivot) instead of using the default ChimeraX center of rotation method.

Original ticket #164g