Tests should write cpl avghist files in addition to instantaneous

_From @billsacks on December 8, 2015 19:22_

Currently out tests write instantaneous cpl hist files at the end of the run. It would be helpful if they also wrote avghist files. Ideally these would be averaged over the whole length of the run (AVGHIST_OPTION = STOP_OPTION, AVGHIST_N = STOP_N).

The point of this is: Right now, your component could send different fields in the middle of a run without resulting in baseline failures.

One caveat with this is that (I think) cpl avghist files do not restart properly. However, if the main purpose of this is for baseline comparisons (which is what I was imagining), then this may not be a serious problem - as long as we don't try to compare the avghist files between the base and restart versions of a test.

_Copied from original issue: CESM-Development/cime#329_