CHANGES:

1. Added `data/example_cases.py`, `data/uk_clip.py`, `covid/summary.py`, `summary.py`
2. `summary.py` computes summary values required by reports to SAGE
3. Small changes to `inference.py` and `simulate.py` to reflect baked-in `nu` parameter in `model_spec.py`.

Removed redundant code from `inference.py` (duplicated in `model_spec.py`). Small change to `xi_freq` parameter in `model_spec.py`.

Changes:

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