More tests for Hertz expressions and bug fixes. Center stress in surface_stress was wrong for nu!=0.5. Fixes some divide by zero problems in full Hertz stress expression. Added Hertz tests (check full stress against surface and centerline stress). Renamed *subsurface_stress* to just *stress* since it now works in the full domain.
Corrected since for subsurface stress in Green's function. Updated test to reflect new name for analytical reference result (Hertz.stress instead of Hertz.subsurface_stress).