Feature: Sum colliding entries rather than overwrite them, during read and consolidation

[dhutchis]

Summing entries into an array, as in A += B, is a common pattern in array and BLAS-like processing. Currently newly written cells simply overwrite old cells with the same coordinates. It would be ideal if a user could specify a function to run that sums together values instead of overwriting them. This function would probably be stored in the array metadata, perhaps accepting something of the form

void* (*plus)(void*, void*)

except that the function code ought to be serializable so that it could be stored with the array and applied to a particular attribute.

Without this feature, the workaround is to read the array first, determine colliding entries with those entries to be written, and then write back the updated entries. It has some performance implications.