Scale some water tracer fields by precip_factor similarly to the corresponding bulk fields

In prep_ice_mod and prep_ocn_mod, a few fields are scaled by a precip_factor:
- In prep_ice_mod:
- Faxa_rain
- Faxa_snow
- Fixx_rofi
- In prep_ocn_mod:
- Faxa_rain
- Faxa_snow
- Foxx_rofl
- Foxx_rofi

I *think* the same scalings should be applied to the corresponding water tracer fields. However, I'm not sure of the purpose of this precip_factor and whether it is still even used. I see that it was set in POP, but doesn't seem to be set in MOM. So before we go to the trouble of adding handling of water tracers here, I'd like to get confirmation that this is needed, as well as confirmation that it should indeed be applied to the tracer fields as well as the bulk fields (probably yes, if this is still actually used).

@mvertens @klindsay28 - do either of you have a sense of this?