Fixes AD NaN issue.

[tpapp]

cf tpapp/DynamicHMCExamples.jl#12

note that the transformation accessors no longer get forwarded, I also
fixed that.

Uses LogDensityProblems v0.6.0 which I just tagged, should trickle
through the registry soon.

[goedman]

Thanks a lot Tamas! Definitely runs to completion now. Will complete several more of the SR (StatisticalRethinking) examples and compare draw density graphs.

I must say, apart from really enjoying doing these examples in DynamicHMC style, using the DynamicHMC package is changing my thinking how to proceed with SR.

Also will restructure SR to make it into a separate Github org with separate packages for the 4 used mcmc solutions (right now it is recompiling SR all the time, just too slow). And it doesn't really belong to StanJulia per se.

[tpapp]

I am happy to hear that you find DynamicHMC useful. Note that there will be an upcoming API reorganization in the near future (2-3 months), see the issue tpapp/DynamicHMC.jl#30 if you would like to suggest anything.