simulate and predictLVs functions

[dwarton]

• drafted a simulate function, and predictLVs.gllvm, to predict lvs for new Ys.
• added a TMBfn to gllvm output so you can access the original function optimised (needed to predict LVs)
• predictLVs needs more bells and whistles, suggest that it be cleaned up a bit and added to predict.gllvm as type="lv" or similar...
• simulate.gllvm comes with no documentation, the intention is that it behave like generic simualte, although optionally we could add a "conditional" input argument which fixes lvs (conditional=TRUE) or simulates them (conditional=FALSE, amounting to glm simulation, unless we simulated from the posterior for the lv...). Currently it only does cond=FALSE.