[wpimath] Move RobotDriveBase::ApplyDeadband() to MathUtil (#3529)

It's a useful function outside of the drive classes.

For backwards compatibility, deprecate (rather than remove) RobotDriveBase.applyDeadband()

wpimath/src/main/native/cpp/MathUtil.cpp

@@ -0,0 +1,23 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "frc/MathUtil.h"
+
+#include <cmath>
+
+namespace frc {
+
+double ApplyDeadband(double value, double deadband) {
+  if (std::abs(value) > deadband) {
+    if (value > 0.0) {
+      return (value - deadband) / (1.0 - deadband);
+    } else {
+      return (value + deadband) / (1.0 - deadband);
+    }
+  } else {
+    return 0.0;
+  }
+}
+
+}  // namespace frc