Add CG Proximity/Contact Check for Auto Contact

[srwopschall]

• Thin plates or shells one to only a few elements thick will erroneously trigger 'opposing side of plate' contact.
• Consider pairs proximate if the face centroids are no further apart (in separation or interpenetration) than the element thickness.
• Alternatively, or additionally, we know 'opposing side of plate' contact will be full face interpenetration and can check nodal positions relative to element thickness.