ENH: single pass on file when filtering by metadata
MAINT: removed old print statement
ENH: by sample_ids is now single pass and not in memory
ENH: by prefix is now single pass and not in memory
resolve issue 1682
self.test_out correction
Fix futures version issue with scikit-bio

See https://github.com/biocore/scikit-bio/issues/691
Merge pull request #1684 from jairideout/futures-version-fix

Fix futures version issue with scikit-bio
Merge pull request #1683 from ekopylova/fix_1682

Fix 1682
Fix #1686
Merge pull request #1687 from antgonza/fix_1686

Fix #1686
Merge pull request #1681 from wasade/issue1506

Issue1506