Add the ability to write out monthly average history fields

Bette is interested in monthly average smb and artm. It is currently non-trivial to add code to write these values to the files (smb is passed to cism annually through the coupler). However, it would be good to see a seasonal cycle for these values in the future. Bill L says:
" Kate and Bill S., let's follow up on this when time permits. I wonder if we could do the interpolation/downscaling of SMB from CLM to CISM monthly instead of annually, but still get the same result (or something close) for the annual average SMB. This is something we'll want anyway, so that we can look at the effects of seasonal meltwater inputs."
Bill S says:
"From talking with Mariana about related issues over the last week, I think this capability of doing averaging monthly rather than annually may get easier with the NUOPC coupler, so it would be good to know what the timeframe is with which this is needed."