get log-density of prior components

[bgctw]

There is already the useful feature of querying the LogLikelihoods of different observations with pointwise_loglikelihoods for a given sample or chain of samples. Please, provide this functionality also for components of the prior.

Background: I want to experiment with tempering, i.e. dividing the log-probability-density by a factor, different components of the prior and the likelihood differently and need to inspect the single components for this.

[penelopeysm]

Closed by #663