ENH: Extend gp.Latent to allow for multiple outputs

[bwengals]

Context for the issue:

Currently gp.Latent can only handle a single GP observation, so f ~ GP(0, k(x, x')). It should be generalized to allow f_i ~ GP(0, k(x, x')), which is multiple observations of the same GP.

It "should" be pretty straightforward to extend all the GP implementations to handle this case, but it's probably most important for gp.Latent.