*SetFieldStiffness() - symmetric field pair
Sets the stiffness properties of a symmetric field pair.
Syntax
*SetFieldStiffness(field_name, LEFT|RIGHT, k1-k36)
    Arguments
- field_name
 - The variable name of an existing field pair.
 - LEFT RIGHT
 - If LEFT is specified, the properties are assigned to the left field, and the right field properties are made symmetric to the left field.
 - k1-k36
 - The linear stiffness for each entry of the 6x6 stiffness matrix.