Support for matrix-valued topic/word prior eta in LdaModel. 

This allows topics to be seeded with specific words. (Or however you like, really. It could e.g. be used to impose a fixed background topic if you specify the prior for one topic to have a specific base measure and a really high concentration)

This is quite a small patch, it looks like the only place which needs changing to handle a matrix for eta is the computation of the variational bound.

Added a test for the seeding behaviour, although like the existing testTransform test it's just a high-level sanity check.