Fix case where var_pooled contains zeros

[tom-osika]

Hi @Jfortin1

Thanks for providing this python package, this has been incredibly useful for some research I'm doing currently.

When adjusting the values of the var_pooled array that are zero, I think it was meant to take the median of the nonzero values of the array, not the median of the intermediate binary array (which would either be 0 or 1).

Let me know if this change is correct, I haven't studied the code extremely in-depth.

Thanks again,
Tom