This commit adds the option to apply a linear decay in the
neutral diffusion fluxes within a transition zone defined
by the boundary layer depths of adjacent columns. This option
is controlled by a new parameter NDIFF_TAPERING, which is only
available when NDIFF_INTERIOR_ONLY=True. By default
NDIFF_TAPERING=False and answers are bitwise identical.

Simplifies and reduces the code by adding hbd to the neutral
diffusion contril structure. This avoid the need to "extract"
hbl multiple times. Answers are bitwise indenticals.