Previously, the Events matrix expected by model.log_prob was ordered
[T, M, X] where T is the number of timpoints, M is the number of
meta-populations and X is the number of transitions.  However, it was
found more convenient to work with [M, T, X] for the purposes of data
augmentation.  This meant extra tf.transpose calls.

model.log_prob now expects [M, T, X], with any further batch
dimensions added as outer dimensions.