transition to MCMCDiagnosticTools for diagnostics

[Tamas K. Papp]

(Cf TuringLang/MCMCDiagnosticTools.jl#41)

Specifically, write a wrapper that exposes a chain or a vector of chains as an AbstractArray. Ideally as lightweight as possible, something cobbled together from existing packages.

Also

• update examples (#165)
• update DynamicHMCExamples (tpapp/DynamicHMCExamples.jl#38)
• update LogDensityTestSuite (decided to keep the current column-based format)