Fix truncation of h2osfc

[billsacks]

From @billsacks on August 25, 2017 21:10

Currently, SoilHydrologyMod.F90: UpdateH2osfc has:

        ! TODO(wjs, 2017-08-22) This is here to maintain bit-for-bit answers with the old
        ! code. If we're okay changing answers, we could remove it. Or maybe we want to
        ! put a more general truncation in here, like:
        !   if (abs(h2osfc(c)) < 1.e-14_r8 * abs(h2osfc_orig)) h2osfc(c) = 0._r8
        ! But I'm not sure what the general philosophy is regarding whether and when we
        ! want to do truncations like this.
        if (truncate_h2osfc_to_zero(c)) then
           h2osfc(c) = 0._r8
        end if

I'll remove this current truncate_h2osfc_to_zero which was just there for backwards compatibility. In its place, I'll do something like the code that is suggested in the comment.

Copied from original issue: NCAR/clm-ctsm#9

[billsacks]

This is fixed.