Mediator average history files should give bit-for-bit results if they span a restart

Mediator history files with time-averaged fields currently do not preserve averages across restarts. This is a problem for https://github.com/esmci/cime/issues/873 , but is also a more general problem, such as for data assimilation runs that have frequent restarts. Currently, no attempt is made to save accumulating mediator fields to a restart history file.