IMP: adds `filter_missing` flag to ancom

[valentynbez]

• if True filters samples with missing metadata
• if False raises en error informing user about missing values
• tests
• test code refractor
• closes #90

[valentynbez]

Not sure what is happening, the command py.test --pyargs q2_composition runs on my machine without errors

[lizgehret]

Thanks for implementing these changes @valentynbez! I've fixed the merge conflicts and removed the logging (QIIME 2 doesn't have the ability to utilize that) - I think there is a small test failure just due to regex matching, but I'll pull this down locally and test on my end to confirm. Otherwise this should be good to go!

[valentynbez]

Thanks for looking into it!

[lizgehret]

I added one small fix just on the ID sorting in your error branch, but this is otherwise good to go! Thanks @valentynbez 🙂