Property "LoadCapacity" should be generalized

[jwouellette]

Currently, I can only find the bearing or load capacity via LoadCapacity in Pset_ElementAssemblyTypeRigidFrame and Pset_CessBetweenRails. But the concept should be more generalized to be applicable to any structural element assembly or object, not as a matter of structural analysis, but as a matter of load checks and design parameters.

Would suggest making it a property in all Pset_XxxxxCommon property sets for structural elements.