Removed contact radius parameter from surface_displacements. Need to normalize radial position before passing to the function. Updated docstrings.
Added new Green's function module to contact_mechanics. This module implements surface Green's function (normal displacements only so far). Added test that compares with Hertz solution.
Added Green's function test to test runner.
Bug fixes to Hertz subsurface stress. zz component passes test.
Bug fix in azimuthal stress.
Green's function for subsurface stress must be zero at r=0.
Added test for subsurface stress (comparing numerical Boussinesq to analytical Hertz/Huber expression).