compute_time_var is broken about esmlab

[matt-long]

That works, thanks. I will need to get to know this code better and perhaps we can discuss how to improve the interface. The original intent was to provide this as a utility for the user.

[andersy005]

@matt-long, because xarray's accessors only accept one argument, which is an xarray.Dataset in this case, I added a somewhat unpleasant hack .esmlab.set_time() to allow passing attributes such as year_offset, time_coord_name to the class' instance.

In order to get compute_time_var to work, try this

ds.esmlab.set_time('time').compute_time_var()

or

ds.esmlab.set_time().compute_time_var()

[andersy005]

In case we don't need to instantiate the accessor with year_offset, time_coord_name attributes, we can remove this hack. Let me know what you think.