Improvements to split_otu_table.py

This PR reduces the code complexity of split_otu_table.py and adds functionality
to allow a user to pass multiple fields to split the biom table on. Adds significant documentation to the library code and the script.
addressed comments
remove unused imports
Merge branch 'master' into split_otu_table_update
Updated ChangeLog.md
Merge pull request #1663 from wdwvt1/split_otu_table_update

Improvements to split_otu_table.py
Fixed bug where removing samples with NA in mapping file changed baseline error estimate.