convert BNN time series forecasting example to blackjax

[murphyk]

Translate this theano/pymc code into blackjax
https://github.com/TeaPearce/Expressive_Priors_in_BNNs/blob/master/code/timeseries_hmc_05.py

This should reproduce fig 3 from te paper below (first 3 columns, ignore the GP part)

T. Pearce, R. Tsuchida, M. Zaki, A. Brintrup, and A. Neely, “Expressive Priors in Bayesian Neural Networks: Kernel Combinations and Periodic Functions,” in UAI, May 2019 [Online]. Available: http://arxiv.org/abs/1905.06076