Added ahal

This is a formatted copy of WPILib's default user-visible C++ API, with
a bit of completely unnecessary functionality stripped out. Most of the
stripping so far is only related to weird threading decisions.

Change-Id: Icbfd949b48cd115561862cb909bcc572aba0e753
diff --git a/frc971/wpilib/ahal/DigitalInput.h b/frc971/wpilib/ahal/DigitalInput.h
new file mode 100644
index 0000000..2053e4e
--- /dev/null
+++ b/frc971/wpilib/ahal/DigitalInput.h
@@ -0,0 +1,48 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2008-2017. 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <stdint.h>
+
+#include <memory>
+#include <string>
+
+#include "frc971/wpilib/ahal/DigitalSource.h"
+
+namespace frc {
+
+class DigitalGlitchFilter;
+
+/**
+ * Class to read a digital input.
+ * This class will read digital inputs and return the current value on the
+ * channel. Other devices such as encoders, gear tooth sensors, etc. that are
+ * implemented elsewhere will automatically allocate digital inputs and outputs
+ * as required. This class is only for devices like switches etc. that aren't
+ * implemented anywhere else.
+ */
+class DigitalInput : public DigitalSource {
+ public:
+  explicit DigitalInput(int channel);
+  virtual ~DigitalInput();
+  bool Get() const;
+  int GetChannel() const override;
+
+  // Digital Source Interface
+  HAL_Handle GetPortHandleForRouting() const override;
+  AnalogTriggerType GetAnalogTriggerTypeForRouting() const override;
+  bool IsAnalogTrigger() const override;
+
+ private:
+  int m_channel;
+  HAL_DigitalHandle m_handle;
+
+  friend class DigitalGlitchFilter;
+};
+
+}  // namespace frc