Integrate Nonius benchmark into Catch2

Changes done to Nonius:
* Moved things into "Catch::Benchmark" namespace
* Benchmarks were integrated with `TEST_CASE`/`SECTION`/`GENERATE` macros
* Removed Nonius's parameters for benchmarks, Generators should be used instead
* Added relevant methods to the reporter interface (default-implemented, to avoid
breaking existing 3rd party reporters)
* Async processing is guarded with `_REENTRANT` macro for GCC/Clang, used by default
on MSVC
* Added a macro `CATCH_CONFIG_DISABLE_BENCHMARKING` that removes all traces of
benchmarking from Catch

include/internal/benchmark/catch_outlier_classification.hpp

@@ -0,0 +1,29 @@
+/*
+ *  Created by Joachim on 16/04/2019.
+ *  Adapted from donated nonius code.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+// Outlier information
+#ifndef TWOBLUECUBES_CATCH_OUTLIERS_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_OUTLIERS_HPP_INCLUDED
+
+namespace Catch {
+    namespace Benchmark {
+        struct OutlierClassification {
+            int samples_seen = 0;
+            int low_severe = 0;     // more than 3 times IQR below Q1
+            int low_mild = 0;       // 1.5 to 3 times IQR below Q1
+            int high_mild = 0;      // 1.5 to 3 times IQR above Q3
+            int high_severe = 0;    // more than 3 times IQR above Q3
+
+            int total() const {
+                return low_severe + low_mild + high_mild + high_severe;
+            }
+        };
+    } // namespace Benchmark
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_OUTLIERS_HPP_INCLUDED