Use correct tracer concentration for irrigation withdrawals from surface (river) water

For the tracer versions of `qflx_irrig_patch` and `qflx_irrig_col`: I'm currently setting the tracer values to a fixed ratio times the bulk fluxes. This is like saying, "We don't know what the tracer / isotope ratio is in the source." Eventually, I think we'll want to set the tracer / isotope ratios of these fluxes based on the actual irrigation source. If there are multiple possible sources, I think we'll want different irrigation fluxes for each possible source – for example, we'll have a `qflx_irrig_from_river_patch` and a separate `qflx_irrig_from_aquifer_patch`, where, for a given patch, one of those will typically be 0 (though I suppose it would be possible for a patch to take some of its irrigation from rivers and some from aquifers). (See also https://github.com/ESCOMP/ctsm/issues/511 for some discussion of wanting separate fluxes when there are separate sources. The main reason is that this will keep the code for setting the tracer version of the fluxes simpler and more uniform - rather than needing conditionals in that code.)

Pinging @davidnoone and @swensosc on this. I don't think this needs to be resolved immediately, but I wanted to make note of it.