esmlab operations clobber existing ds.attrs['history'] values

[klindsay28]

Is your feature request related to a problem? Please describe.
The following commands

ds = xr.open_dataset('/glade/work/klindsay/analysis/CESM2_coup_carb_cycle_JAMES/tseries/FG_CO2_ocn_piControl_00.nc')
print(ds.attrs['history'])
ds_ann = esmlab.resample(ds, freq='ann')
print(ds_ann.attrs['history'])

yield the output

created by /glade/work/klindsay/analysis/CESM2_coup_carb_cycle_JAMES/tseries_mod.py at 2019-10-24 20:32:05 UTC

and

2019-10-30 15:28:45.220584 esmlab.resample(<DATASET>, freq="ann")

Describe the solution you'd like
I think it would be useful to prepend to existing history content, instead of overwriting it, if it exists.

This behavior would be consistent with NCO.