Remove test failing comma from generated_by string in BIOM

Addresses #71

Note that after this bug is fixed in BIOM the original generated_by
string can be used and tested.
Update warning and option usage for low/high memory machines for predict_metagenomes.py
Merge pull request #73 from mlangill/doc_memory_usage

Update documentation for metagenome prediction tutorial.
Merge pull request #72 from mlangill/issue71

Remove test failing comma from generated_by string in BIOM
Merge branch 'master' of https://github.com/picrust/picrust