Remove unnecessary import
Two evaluation bugs: Perplexity split for doc-topics and randomising input order

Where we have to use words to infer topics for a document when
evaluating perplexity, split the document so that half the words are
used to infer topics and half to evaluate perplexity

Also as cross-val segments are not randomised, randomise the data
first. This helps when documents appear in the corpus sorted by topic,
so if we trained on the first 80%, we would have never seen the topic
that appeared in the last 20%