Bug in iCAM5 rain evaporation water tracer code

[nusbaume]

There is a bug in the water tracer/isotope deep convective precipitation evaporation scheme (wtrc_precip_evap) where a mass fixer applies the rain ratio to the total (rain+snow) precipitation mass, when it instead should apply it solely to the rain mass. This bug appears to have no effect on the water isotope delta-values (as the ratio cancels the error out), but can occasionally influence the deep convective precipitation for water tracers/tags in locations where convective snow occurs, especially at high latitudes.

Thus, although unlikely to meaningfully impact science results, this bug should still be fixed when possible to ensure maximum simulation fidelity.