1. Formatting -- black style!
2. After profiling, implemented a better workaround for the
tfd.Categorical bug.  Replaced with an inherited Categorical2 class,
with overloaded log_prob function.
3. Fixed a  bug in EventTimeProposal where tfd.FiniteDiscrete was
being used on a non-strictly-increasing outcomes vector.
4. Increased number of meta-populations to 149 to mirror UK UTLA regions.