helper to display torsion angle values.

constructor:

   Dihedral(as0,as1,as2,as3)

      as[0-3] are <m atom>.Atom objects, or strings or <atomSel>.AtomSels
   containing single atoms

methods:

   value(): 

      return the value of the dihedral angle (in radians)

   derivs(): 

      return the value of the gradient of the dihedral angle for the four 
    atoms. The return value is a tuple:

      (dPhidAtom0,dPhidAtom1,dPhidAtom2,dPhidAtom3)

   atom0(), atom1(), atom2(), atom3():

      return the requested atom.

The dihedral angle is the angle about atoms a2 and a3 below:

                              a3
                             /
                        a1-a2
                       /
                     a0