Add CN prec. control call to fix problems related to small neg. values

Small negative values (roughly roundoff-level different from zero) in
frootc (and possibly other quantities) were occasionally creating
problems with carbon isotope fluxes and FPI in the first time step of
the year, at the time of transient landcover change. This tag fixes the
problem by introducing an extra call to SoilBiogeochemPrecisionControl
in between computing the patch-level transient landcover fluxes and
moving these to column-level. In particular, this truncates small
negative values of decomp_cpools_vr_col to zero, which prevents the
previous blow-ups.

For most of the problematic fields, the explanation seems to be: frootc
can sometimes be negative; this is intentional. Negative frootc causes
negative dwt_frootc_to_litter if the patch in question is shrinking. The
resulting negative fluxes cause problems in the ciso calculation. This
can be worked around by inserting an extra precision control call
between the calculation of the dwt fluxes and the ciso fluxes, so that
small negative dwt fluxes are set to 0.

This does not necessarily fully explain the issue with FPI, but the
insertion of the extra precision control call fixes that issue, too.

For more details, see the discussion in
#741

Resolves #741