Bug fix (bug #44)

Doug Jacobsen points out that MPAS-O differentiates between total number of
vertical levels and active number of vertical levels... so the array
CVmix_vars%Mdiff_iface may have more than CVmix_vars%nlev+1 elements. The
subroutine cvmix_coeffs_kpp_low did not consider the situation where Mdiff_out
was a smaller array than old_Mdiff.