DihedralTraPPE

class DihedralTraPPE : public feasst::BondFourBody

See the Dihedral section of http://trappe.oit.umn.edu/

\(U(\phi)=c_0+c_1[1+\cos(\phi)]+c_2[1-\cos(2\phi)]+c_3[1+\cos(3\phi)]\)