Cleanup inheritance chains

The elements inheritance chains are revised in order to get rid of diamond inheritance chains and the ensuing need to use virtual public inheritance. ElemWithDof and ConstitutiveLawOwner classes are gone. The node elements still have to be reworked, possibly following the scheme devised for the body AD elements. Elements (e.g.rod) should not be derived from ConstitutiveLawOwner; rather, they should own it;