Kahan Addition

[Matthias Boehm]

Hi all,

experiments with lower precision levels and HW with higher memory 
bandwidth have shown that our use of Kahan Addition becomes increasingly 
costly. In the past, the overhead of computing corrections was hidden by 
I/O and memory bound-operations like unary aggregates (e.g., rowSums, 
colSums, sum) but this is changing.

So, for reasons of performance and code complexity, I intend to entirely 
remove Kahan addition from our system (in unary aggregates and partial 
aggregation). If there are objections or concerns, let's discuss them.

Regards,
Matthias