[BUG] Sum inaccurate result

[tfeher]

Describe the bug The sum kernel does not handle underflows correctly, and that leads to inaccurate results.

Steps/Code to reproduce bug

As reported by @lijinf2:

We also did an experiment of summing up a column of 0.1 across 130,382,757 rows. Observed raft returns 12,528,200.

Expected behavior Compensate for underflow e.g. by Kahan summation or a similar method.