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.

src/UnitTests/DistributionTests/Discrete/CategoricalTests.cs

@@ -155,6 +155,22 @@ namespace MathNet.Numerics.UnitTests.DistributionTests.Discrete
             Assert.Throws<ArgumentOutOfRangeException>(() => b.P = _badP);
         }
 
+        /// <summary>
+        /// Validate mean.
+        /// </summary>
+        /// <param name="p">An array of nonnegative ratios.</param>
+        /// <param name="mean">Expected value.</param>
+        [TestCase(new double[] { 0, 0.25, 0.5, 0.25 }, 2)]
+        [TestCase(new double[] { 0, 1, 2, 1 }, 2)]
+        [TestCase(new double[] { 0, 0.5, 0.5 }, 1.5)]
+        [TestCase(new double[] { 0.75, 0.25 }, 0.25)]
+        [TestCase(new double[] { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }, 5)]
+        public void ValidateMean(double[] p, double mean)
+        {
+            var n = new Categorical(p);
+            AssertHelpers.AlmostEqual(mean, n.Mean, 14);
+        }
+
         /// <summary>
         /// Can sample static.
         /// </summary>