Added unit tests to validate the following properties of the
Categorical distribution:
- Variance,
- StdDev, and
- Median.
TODO: Find out the expected behaviour of Median in ambiguous cases
(e.g. where P(X=1)=0.5, and P(X=2)=0.5), then add appropriate unit
tests.
TODO: Research how to calculate entropy, then add unit tests for the
Entropy property.