Get rid of set_* arguments in hist_addfld calls

[billsacks]

Although this is still open to discussion (I'll open a discussion issue shortly), I propose getting rid of the set_* arguments in hist_addfld calls. In cases where we have set_=spval, we're going to use landunit_mask (#1347). In places where we have set_=0, my preference is to do this setting in initCold instead: I find it unintuitive to set fields to certain values in the hist_addfld call, and I'm also concerned that doing the setting in the hist_addfld call can be misleading because this could be overridden later in the code (i.e., it is not guaranteed that the field will remain at the value set via the set_* argument).