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.
typo resolved. 

renamed to ```make_field_value_list```. the sorting keeps order correct for other steps and avoids a bunch of calls to sorting later.