Add time_bounds to history files

From @sherimickelson

> Do you know if there were plans to add a time bounds variable in the cism output files? It seems as if cism is the only component that doesn't do this and this variable is being requested by cmip6. I could automatically figure this out, but I can see it being error prone because there will have to be a lot of assumptions coming from my code that will likely break if any conditions change. It seems like it would be more reliable coming from the model itself. Would it be possible to add this in for the 2.1 release?