Cumulative normalization, wrong variance computation

[zmolikova]

Hi,
I suspect that the computation of variance in CumLN is missing a division by the length of the sequence. That is:

asteroid/asteroid/masknn/norms.py

Line 101 in 140733b

 cum_var = cum_pow_sum - cum_mean.pow(2) 

should be
cum_var = cum_pow_sum / cnt - cum_mean.pow(2)
as in $var(x) = E[x^2] - E[x]^2$.

[Pariente Manuel]

You're fully right, thank you !

Could you please submit a PR ?