Expand view_reduction to handle array of accumulators.

[ambrad]

Is your feature request related to a problem? Please describe.
view_reduction currently handles just a single Pack as accumulator. The result is that in SHOC, there are two cases where view_reduction is called multiple times in a row on different Packs.

Describe the solution you'd like
view_reduction's output argument result can be an array of Packs.