1. Corrected bug in UncalibratedEventTimesUpdate.one_step, and factored out some propagation functions.
2. Inserted a tf.cond into event times update step to deal with the case where there is no preceding or next event (e.g. S->E in an SEIR model doesn't have a preceding event).  This has reduced performance, and could possibly be replaced with a NOOP calculation instead.
3. Removed call to MH_within_Gibbs -- this proved too restrictive when requiring more than one update to a state_part per sweep of the MHwG sampler.