GSDMM class consumes the docs and vocab class

[rwalk]

The constructor for GSDMM with signature:

pub fn new(alpha:f64, beta:f64, K: usize, maxit:isize, vocab:HashSet<String>, docs:Vec<Vec<String>>) -> GSDMM

consumes the docs and vocab objects. This means that user will need to make copies of the data if they wish to use either of these vectors in subsequent processing.

Should be simple to refactor the interface to borrow these instead.