Fixed comment in TriangularDistribution example

The <summary> tag of the TriangularDistribution class read "ContinuousUniform distribution example". It now reads "Triangular distribution example".
ValidateMean unit test for Categorical dist

Unit tests to validate Categorical.Mean. Currently they fail, indicating
that Categorical.Mean is not correctly implemented.
Unit tests for StdDev, Variance, Entropy, and Median properties are
still required.
Fix to Categorical.Mean property.

Categorical.Mean was being calculated as _pmfNormalized.Mean(), which
 is actually the mean probability across all possible values, rather
 than the mean value of the distribution.
I've replaced it with a loop which performs the following standard
 calculation for mean value:

mean = Sum(x * f(x), x=0..N)
where f(x) is the probability mass function, and N is the maximum
 value.

An alternate method of calculation is:
 Mean = Sum( 1 - F(x), x=0..N) or similar[1].
That might be preferable if it results in less computational error,
 but it's a bit obscure, so for clarity I decided to go with the
 standard calculation for mean value.

[1]: https://en.wikipedia.org/w/index.php?title=Expected_value&oldid=585750484#Discrete_distribution_taking_only_non-negative_integer_values
Categorical dist: Tests for Var, SD, and Median

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.
Merge remote-tracking branch 'david-in-perth/triangular-distribution'
Merge remote-tracking branch 'david-in-perth/categorical-distribution'
Distributions: Categorical impl of mean, variance, stddev and median #187
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