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.
Can you note these changes in the ChangeLog? i. e. note that this script changed from --otu_table_fp to --biom_table_fp.