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't chop up the biom table because we are iterating through it taking pieces at a time. If we did ```inplace=True``` we'd cut it down to the first sample set to come through.