Catch2 unit testing capabilities and alias sampling unit test

tests/cpp_unit_tests/test_distribution.cpp

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+#include "openmc/distribution.h"
+#include "openmc/random_lcg.h"
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("Test alias method sampling of a discrete distribution")
+{
+  int n_samples = 1e6;
+  double x[5] = {-1.6, 1.1, 20.3, 4.7, 0.9};
+  double p[5] = {0.2, 0.1, 0.5, 0.05, 0.15};
+  double samples[n_samples];
+
+  // Initialize distribution
+  openmc::Discrete dist(x, p, 5);
+  uint64_t seed = openmc::init_seed(0, 0);
+
+  // Calculate expected distribution mean
+  double mean = 0.0;
+  for (size_t i = 0; i < 5; i++) {
+    mean += x[i] * p[i];
+  }
+
+  // Sample distribution and calculate mean
+  double dist_mean = 0.0;
+  for (size_t i = 0; i < n_samples; i++) {
+    samples[i] = dist.sample(&seed);
+    dist_mean += samples[i];
+  }
+  dist_mean /= n_samples;
+
+  // Calculate standard deviation
+  double std = 0.0;
+  for (size_t i = 0; i < n_samples; i++) {
+    std += (samples[i] - dist_mean) * (samples[i] - dist_mean);
+  }
+  std /= n_samples;
+
+  // Require sampled distribution mean is within 3 standard deviations of the
+  // expected mean
+  REQUIRE(std::abs(dist_mean - mean) < 3 * std);
+}