Added usearch61 chimera checking (denovo and ref) to identify_chimeric_seqs.py and associated unit tests.  Two related functions added to pycogent_backports/usearch.py along with associated unit tests.  Slight modification to qiime/util.py for fasta splitting function on SampleIDs to return a list of output filepaths.
Added tutorial section for usearch61 chimera checking
Added comment to changelog regarding tutorial addition for identifying chimeric sequences and usearch61
Fixed conflict in scripts/identify_chimeric_seqs.py (authorship list)
Fixed conflict with authorship list in scripts/identify_chimeric_seqs.py
Modified usearch61 denovo chimera check to just look for Y and N chimera flags, as some system-dependent randomness affects other numbers for uchime output file.
Modified identify_chimeric_seqs.py script to have single credits line with all authors, modified script example with usearch61 to not use split_by_sample option to avoid error when output file already exists.
Merge branch 'usearch61_chimera_tutorial_change1' into usearch61_chimera_implementation
Fixed merge conflict with Changelog
Merge pull request #943 from walterst/usearch61_chimera_implementation

Added usearch61 chimera checking (denovo and ref) to identify_chimeric_s...