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.

1. Fixed bug in covid.impl.event_time_mh._reverse_move where Multinomial one_hot tensor wasn't being advanced for the reverse move.  This is a temporary workaround while tf.nn.sparse_softmax_cross_entropy_with_logits is fixed.

2. Batched EventTimesProposal to move multiple meta-populations.

3. Batched _move_events to allow for multiple meta-population updates

4. Refactored covid.impl.event_time_proposal._abscumdiff so we calculate `bound_times` outside the function.  This allows clipping of times proposed outside [0, T] to [0, T], thus avoiding an error in tf.gather_nd in CPU mode.  The output of EventTimesProposal.sample and EventTimesProposal.log_prob might be inconsistent with what we expect, but that's okay as the move gets rejected as an out-of-bounds proposal anyway.

1. Edited _move_events to take into account metapopulation selection.
2. Replace bound_t.ndim with bound_t.shape.rank for TF Graph Mode running.
3. Revised index calculation in `_abscumdiff`.  Re-wrote test to accommodate.
4. Code rationalisation in FilteredEventTimesProposal to return a simpler data structure.
5. Changed dimension of results.extra field to trace `x_star`.
6. Edited tests in `tests.test_mcmc.py` for new `_abscumdiff` interface.

1. Wrote wrapper `FilteredEventTimesProposal` for EventTimesProposal to subset metapopulations.  This currently selects just one meta-population, but the plan is to select more.
2. Moved test fixture to folder inside tests.
3. Wrote (failing) TestFilteredEventTimesProposal test.

- flatten out the distribution of delta_t; no longer a
JointDistributionNamed, just a FiniteDiscrete
- replaced OneHotCategorical with a Categorical (sample needs an expand dims
after)
- fix bug -- were using zeros instead of int max's for upper bounds
- thread dtypes through various functions
- inlined some arg-free distributions in JointDistributionNamed
- minor reformatting to appease Chris Suter's formatting compulsions
- pprint in tests; fix test data paths (weren't working for Chris
Suter's setup)