Issue title: Diagnosing HMC: Energy Distribution
 Issue body: To better diagnose HMC, in particular the choice of kinetic energy and adaptation thereof, it may be worthwhile to provide metrics that compare the marginal and conditional (on a particular location) energy distributions. As explain on p. 28 in [1], mismatch between the two may greatly negatively impact performance of the sampler. A simple metric to compute is outlined in Section 6.1, also in [1]. 

[1] https://arxiv.org/pdf/1701.02434.pdf
 Comments: 
 Comment 0: Closed by https://github.com/TuringLang/AdvancedHMC.jl/pull/5
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