+Add Laplacian diffusion of interface heights

  Added the capability to do Laplacian diffusion of interface heights without
regard to the density of layers.  This is accomplished by the new internal
subroutine add_interface_KH, and is controlled by the new runtime parameters
KH_ETA_CONST and KH_ETA_VEL_SCALE.  The new interface diffusivities are subject
to CFL bounding when combined with the previous isopycnal slope diffusivities,
with the two diffusivities reduced proportionately when the CFL limits are
exceeded.  Also simplified many openMP directives in MOM_thickness_diffuse.F90,
and fixed a number of spelling errors or other problems in comments, including
adding units to some variables.  By default, all answers are bitwise identical,
although there are new runtime parameters a new internal subroutine.