Condition number of the sum function.
s = accsum_sumcond ( x ) [s,c] = accsum_sumcond ( x )
a m-by-n, matrix of doubles
a 1-by-1, matrix of doubles, the sum
a 1-by-1, matrix of doubles, the condition number
Condition number of the sum function.
"Stability and numerical accuracy of algorithms", Nicolas Higham
"Handbook of Floating Point Computations", Muller et al