Use post_product_u for momentum budget diagnostics

  Use the new post_product_[uv] and post_product_sum_[uv] routines to simplify
the code calculating derived diagnostics of the momentum budget in
step_MOM_dyn_split, calculate_diagnostic_fields(), horizontal_viscosity(),
vertvisc(), and layered_diabatic().  Also relocated the units for several
diagnostics to the same lines as their conversion factors in some calls to
register_diag_field, to make it easier to spot diagnostics with inconsistent
reported units, and removed variables that are no longer used.  All answers and
diagnostics are bitwise identical.