Oldroyd-B viscoelasticity extended from Su et al. PRL (2013)
-Header file SuViscoelasticity.h defining the Su class which calculates a force field due to Oldroyd-B polymer stress
-SuViscoelasticityTest.cpp and Su.prm test case which uses "Poiseuille" flow. Tests against theoretical steady state and viscoelastic overshoot reference data.
@kuron Please take a look and let me know what needs to be updated