Add MedianFilter class for moving-window median (#2136)

This kind of filter is extremely useful for signals that are susceptible to sudden
outliers - ultrasonics, 1-D LIDAR, and results from vision processing are all
good use-cases.

This also modifies the existing ultrasonic examples accordingly.

wpilibc/src/test/native/cpp/MedianFilterTest.cpp

@@ -0,0 +1,55 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "frc/MedianFilter.h"
+#include "gtest/gtest.h"
+
+TEST(MedianFilterTest, MedianFilterNotFullTestEven) {
+  frc::MedianFilter<double> filter{10};
+
+  filter.Calculate(3);
+  filter.Calculate(0);
+  filter.Calculate(4);
+
+  EXPECT_EQ(filter.Calculate(1000), 3.5);
+}
+
+TEST(MedianFilterTest, MedianFilterNotFullTestOdd) {
+  frc::MedianFilter<double> filter{10};
+
+  filter.Calculate(3);
+  filter.Calculate(0);
+  filter.Calculate(4);
+  filter.Calculate(7);
+
+  EXPECT_EQ(filter.Calculate(1000), 4);
+}
+
+TEST(MedianFilterTest, MedianFilterFullTestEven) {
+  frc::MedianFilter<double> filter{6};
+
+  filter.Calculate(3);
+  filter.Calculate(0);
+  filter.Calculate(0);
+  filter.Calculate(5);
+  filter.Calculate(4);
+  filter.Calculate(1000);
+
+  EXPECT_EQ(filter.Calculate(99), 4.5);
+}
+
+TEST(MedianFilterTest, MedianFilterFullTestOdd) {
+  frc::MedianFilter<double> filter{5};
+
+  filter.Calculate(3);
+  filter.Calculate(0);
+  filter.Calculate(5);
+  filter.Calculate(4);
+  filter.Calculate(1000);
+
+  EXPECT_EQ(filter.Calculate(99), 5);
+}