Modified pycogent_backports/usearch.py to correctly pass HALT_EXEC for usearch_qf (5.X) code.  Also fixed typo in scripts/pick_otus.py
summarize_taxa.py provides an informative error message on no observation metadata

this fixes #219
Modified check_id_map.py to 1.  allow suppression of html generation and 2.  add warnings for empty fields before the Description column
updated based on feedback from @jrrideout
Reworded script text to (hopefully) be clearer.

Fixes #907.
Merge pull request #924 from walterst/check_id_map_columns

Check id map columns
Merge pull request #929 from jrrideout/issue-907

Reworded script text to (hopefully) be clearer.
Merge pull request #922 from gregcaporaso/issue-219

summarize_taxa.py provides an informative error message on no obs metadata