Modify SHOC to avoid double counting of aerosol tracers and cloud droplet number

[singhbalwinder]

Comments by @kaizhangpnl :
"The vertical diffusion of aerosol tracers and cloud droplet number needs to be treated with care. Vertically diffusion of (interstitial aerosols and cloud droplet number) tracers is supposed to be done in SHOC by default, but the "ndrop" routine will also diffuse the aerosol tracers and cloud droplet number as in MAM/E3SM. According to our discussion in the EAGLES MAMxx meeting, it's better to keep the vertical diffusion calculation (for interstitial aerosols and cloud droplet number) in "ndrop" and turn it off in SHOC for initial porting and testing. (Note that the vertical diffusion of cloud-borne aerosols is considered in "ndrop", but not in SHOC since cloud-borne aerosols are not in the tracer array). "