Add time_bounds to history files

[billsacks]

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?

[billsacks]

From @sherimickelson

I added a work around in the postprocessing code to handle this. It brings in an extra dimension (of size 2) and then computes the bounds. So it's up to you if you want to add it, the CMIP6 postprocessing is no longer dependent of it being in the output files.

Thank you, Sheri!

It's probably still good if we do this at some point, but Sheri's help makes this not time-critical.