Separate computing global sums / means from plotting them

[mnlevy1981]

The following block in summary_plot_global_ts() has nothing to do with plotting and should be its own function:

spatial_op = diag_metadata.get("spatial_op", "average")
if spatial_op == "average":
    to_plot = da_weighted.mean(dim=reduce_dims)
    to_plot.attrs = da.attrs
    if "display_units" in diag_metadata:
        to_plot = conv_units(to_plot, diag_metadata["display_units"])
if spatial_op == "integrate":
    to_plot = da_weighted.sum(dim=reduce_dims)
    to_plot.attrs = da.attrs
    to_plot.attrs["units"] += f" {weights.attrs['units']}"
    if "integral_display_units" in diag_metadata:
        to_plot = conv_units(
            to_plot,
            diag_metadata["integral_display_units"],
            units_scalef=diag_metadata.get("integral_unit_conv"),
        )

Besides separating computation and plotting, if it is pulled out then it can then be wrapped in xpersist and cached (if so desired).