Rename for consistency: sum → sum_axis, scalar_sum → sum.

[bluss]

I think the sum_axis is a good naming scheme (see map_axis, fold_axis).

• Add new names sum_axis, mean_axis
• Rename scalar_sum → sum.

[bluss]

Will use deprecations to the extent possible (not 100% since we swap names)

[bluss]

Only alternative I could see would be to use a more evocative naming like map_across, sum_across?

[bluss]

Fixed by #520