Robustness of time-average CISM output fields

CISM nominally supports time-average ('tavg') versions of its output fields. The tavg fields are computed as averages over all time steps between history output intervals (as opposed to snapshots from a single time step, which are written by default). In standalone CISM, it is not possible to correctly write the same tavg field to more than one output file. In coupled CESM (at least, in TG cases to date), tavg fields are erroneously zeroed out for reasons that are unclear. Robust tavg capability is needed to support detailed mass balances, among other applications.