VNT in pointwise logdensities

[penelopeysm]

note that this also requires passing templates into observe statements

[penelopeysm]

passing templates into observe statements

That is actually stupidly annoying, because conditioned variables can generate observe statements, and conditioned variables don't always have a good template

[penelopeysm]

a problem for another day

[penelopeysm]

actually I'm being silly and it's okay for templates because any conditioned index variable will also have a template in the main model body so we are all good. This should be straightforward to implement probably, I can do it tomorrow

[penelopeysm]

Closed by #1279