Improve `@addlogprob!`

[devmotion]

I'm mainly concerned about the, in most cases, incorrect results of logprior currently - which is the main reason for this annoying leafcontext approach. I think this could be fixed by identifying it with a custom observe statement instead of an arbitrary modification of the log probability. One could eg. change the macro syntax to

@addlogprob! val [assume=false] [observe=true]

Then it would still be possible to unconditionally modify the log probability but one could also limit it easily to evaluations that include the likelihood or the prior.

Originally posted by @devmotion in #389 (comment)

[penelopeysm]

Closed by #885