potential division by zero in pChl_subcol computation

[klindsay28]

The computation of pChl_subcol, in marbl_compute_autotroph_photosynthesis, is

            ! GD 98 Chl. synth. term
            pChl_subcol = autotrophs(auto_ind)%thetaN_max * PCphoto_subcol / &
                 (autotrophs(auto_ind)%alphaPI * thetaC * PAR_avg(subcol_ind))

The denominator can underflow to zero, even if the individual terms are positive. This happened in a coupled run using cesm_pop_2_1_20160222 (before MARBL was separated out). In this run, PAR_avg was less than 10^-300, in an individual sub-colum at a single grid point, but it was positive.

A work-around is to set PAR%interface to zero if it is below a threshold, after it is computed and before it is used to set PAR%avg. This is in marbl_compute_photosynthetically_available_radiation.