Added new ContactGeometry method calcSurfacePrincipalCurvatures() for determining min/max curvature and directions at a point on an arbitrary implicit surface.
Justin Si found a better link for the Zhihong paper.
Yuck -- a tab sneaked into the code and caused it to misformat.
Fix principal curvature directions at umbilical points.
Merge pull request #231 from simbody/addPrincipalCurvatureMethodForImplicitSurfaces

Add method for principal curvatures and directions at point on arbitrary implicit surface