Need array of reduction variables

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I have data in a 3-D array indexed by time and two spatial dimensions. When running, I need to periodically report data reduced from this 3-D array. The problem definition can specify an arbitrary number of reference regions where separate statistics need to be calculated. Each reference region defines a subset of the spatial domain where the end user wants a separate set of summary statistics calculated.
The number of reference regions is not known at compile time, so I would like to make an array of ReduceSum<> variables at runtime. This doesn't seem possible because ReduceSum has an explicit constructor, and I can't figure a way to make a container (array, vector, ...) of ReduceSum variables.

Start by examining ReduceSum and its explicit constructor, then review how reduction variables are currently created and stored. Determine whether a runtime-sized array or vector of ReduceSum variables can support separate statistics for an arbitrary number of reference regions; done means the requested container use case is supported and documented by a validating test.

Written by the indexing model from the issue text.