FMT tutorial improvements.

[antgonza]

Improvement Description
Once #353 is merged, we will need to fix/add these things, please add anything else:

• filtering feature table qza: sounded like doing this before ancom to
  tests pre/post fmt will be a good place to show this - currently done
  over healthy/pre/post (@antgonza)
• longitudinal analysis: AFAIK Nick will lead this. (@nbokulich)
• better explain animations_gradient and animations_subject, and include an animation section
• remove not tutorial like columns and add a few lines about finding the full metadata in qiita
• paired ANCOM tests to see what changes pre/post FMT?
• figure out float division by zero in dada2
• no release of taxonomy files
• [add]

Comments
cc: @gregcaporaso , @nbokulich , @thermokarst, @rob-knight

References
#353