Add a balanced version of sum
Add a balanced version of sum

For floating point numbers of about equal value, this is a numerically
more stable way of summing them up.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Oleg Ponomarev <c100474c6090841e9d55cde74d38d81efd269b8d@google.com>

mbakke/ganeti,andir/ganeti,leshchevds/ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti,andir/ganeti,leshchevds/ganeti,ganeti/ganeti,onponomarev/ganeti,andir/ganeti,onponomarev/ganeti,ganeti/ganeti,ganeti/ganeti