Fixed existing cheap K6ROT that's dof6 springs to ground

[victorkemp]

Fixes issue #165

Information for the manual:

• Applies to all CQUAD4 and CTRIA3 elements.
• At each node of the element, it adds a torsion spring to ground about the element normal.
• For QUAD4 elements, it uses the normal to the mean plane even if the element is warped.
• On grid points where SNORM is defined, it uses the SNORM normal. Here, it works well as an AUTOSPC alternative even with large values of K6ROT because that's a singularity DOF which is harmless to constrain. (SNORM is undocumented so this statement should be too)
• It's different from other Nastrans which connect that spring to translational DOFs of adjacent nodes instad of ground.
• The spring's stiffness at each node is
  QUAD4: K = 1e-5 * area * thickness * in-plane shear modulus
  TRIA3: K = 1e-5 * area * thickness * in-plane shear modulus * 2
• These values lead to drilling DOF stiffness consistent with MSC Nastran's K6ROT.
• It does not improve accuracy for curved shells like it does in other Nastrans.
• At grid points shared by more multiple shell elements that are not coplanar, there is no singularity so K6ROT introduces an artifical elastic constraint on the other rotation DOFs.
• Because of that, you should set it to the smallest possible value. 1.0e-4 seems to be OK in many cases, not 1.0e+2 which is the default for MSC Nastran.
• Its intended purpose is to constrain drilling DOFs if AUTOSPC is turned off.
• Default value is 0.