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.
Done, thanks. I was wondering about this one myself. It returns a len 1 tuple but the representation 
```python
(array([1,2,3,4],)
```
seems to indicate 2d-ness?