NUTS Adaptive Metric

[JamesYang007]

• [x] Refactor NUTS
• [x] Adaptive stuff should be abstracted out
• [x] Implement Welford's algorithm
• [x] Implement Hamiltonian for diagonal metric
• [x] Allow users to set options for adaptiveness?
• [x] Make configuration struct to pass into NUTS

[JamesYang007]

30

[jacobaustin123]

closed by 481ce719cf99.