Changes & new features for discrete Thompson sampling:
+ signature of `sample_batch` changed:
+ samples must be passed as `(C, S)` or `(C, ?)` instead of (S, C). This is to permit unequal sample sizes.
+ a new kwarg `correlated:bool` must be specified to choose between jointly or independently sampling from the candidate posterior samples
+ brute-force probability calculation `get_probabilities` is replaced with `sampling_probabilities` that calculates the Thompson sampling probability for each candidate exactly.