The RBE3 Fix

[Bruno02468]

This update is meant to significantly improve the RBE3 element implementation. The fix was achieved with two main changes to the RBE3_PROC code:

• 7d2f4bc and 7d2f4bc introduce new terms to the R_MG matrix, the "S-terms" SXY, SZX and SYZ which lie in the lower-right partition of the R_dd matrix (refer to the manual, last page of Appendix E).
• 09b0b13 changes the way R_MG is constructed so that per-DoF weights are used and eliminates divison by weight totals, preventing divide-by-zero errors.