*+Revise horizontal avg in Calc_kappa_shear_vertex

  Revised the Calc_kappa_shear_vertex code where it averages the diffusivities
at the vertices back to the tracer points when there is either a geometric mean
being used or thickness weighting.  With these changes, the code should now pass
dimensional consistency testing, work properly with openMP threading enabled,
and give sensible values of the diffusivity for massless layers.  Also corrected
the indenting of several recently added lines to follow the MOM6 2-space
indenting convention.  This commit can change answers slightly in some cases
where VERTEX_SHEAR = True, especially if VERTEX_SHEAR_GEOMETRIC_MEAN = True or
VERTEX_SHEAR_THICKNESS_MEAN = True.  The recently added runtime parameter
VERTEX_SHEAR_KD_MEAN_ANSWER_DATE (which has not yet been added to the dev/gfdl
branch of the code) was no longer needed and has been eliminated.

Author: Hallberg-NOAA