Added automatic W threshold

I added an automatic W selection to reject null hypothesis, although the author of original paper stated that they typically recommend the threshold of 0.7 according to https://forum.qiime2.org/t/ancom-low-w-taxa-identified-as-significant-issues-workaround-ancom2-code-instructions/6040.

I added an automatic W selection to reject null hypothesis, although the author of original paper stated that they typically recommend the threshold of 0.7 according to https://forum.qiime2.org/t/ancom-low-w-taxa-identified-as-significant-issues-workaround-ancom2-code-instructions/6040.

Hi,

Thanks for using ANCOM! I apologize for not being able to get back to you sooner. Things have become more hectic by the minute as the week has progressed.

Could you please provide me with the mathematical formula you used to implement the automatic W selection? As you may know, I am not the author of ANCOM, I shall inform them before making substantial changes to the algorithm.

Thank you,

Huang