Make mcmc and warmup also return logdensity_vector.

[tpapp]

Fixes #193.

TODO:

• tests
• documentation

[tpapp]

@astrozot, this is a quick fix, I will finish this PR after the weekend (I am away from my computer), but you can use it. mcmc now returns a logdensity_vector, for the log densities. Until this is merged and released, just

pkg> add DynamicHMC#tp/log-density-in-tree-statistics

in your project. Let me know if I can help with anything else.

[astrozot]

Thank you again @tpapp: I will wait for your merge, as I need to fix a few other things in my code anyway...

[tpapp]

@astrozot: Just a heads-up, I am merging this, but changed the name to logdensities.