Changes:

1. Created a TFP JointDistribution to represent full probability model;
2. Renamed CovidUKStochastic --> DiscreteTimeStateTransitionModel;
3. DiscreteTimeStateTransitionModel now inherits from tfp.Distribution.

Re-wrote discrete_markov_log_prob to batch the call to
tfd.Multinomial.log_prob to avoid lgamma time.

Implemented custom nametuple for tracing innerd of UncalibratedEventTimesUpdate

First pass implementation of data augmentation MCMC.  This is slow, buggy, and possibly not correct yet!  Use with caution!

Got stochastic model fitting with new bounded convergence respecting adaptive proposal.  However, XLA appears now very slow.

1. chainbinom* file and methods renamed to discrete_markov* files and methods.

2. Implemented batched hazard rate calculation, adding functions to automatically broadcast rates to the Markov transition rate matrix.

3. Implemented test simulation and MCMC algorithm.