Add menu options for using cv input as Clock Run/Reset (#25)

Reviewed-on: https://git.pinkduck.xyz/awonak/libGravity/pulls/25

src/analog_input.h

@@ -19,6 +19,8 @@ const int CALIBRATED_HIGH = 512;
 
 class AnalogInput {
    public:
+    static const int GATE_THRESHOLD = 0;
+
     AnalogInput() {}
     ~AnalogInput() {}
 
@@ -74,6 +76,18 @@ class AnalogInput {
      */
     inline float Voltage() { return ((read_ / 512.0) * 5.0); }
 
+    /**
+     * Checks for a rising edge transition across a threshold.
+     *
+     * @param threshold The value that the input must cross.
+     * @return True if the value just crossed the threshold from below, false otherwise.
+     */
+    inline bool IsRisingEdge(int16_t threshold) const {
+        bool was_high = old_read_ > threshold;
+        bool is_high = read_ > threshold;
+        return is_high && !was_high;
+    }
+
    private:
     uint8_t pin_;
     int16_t read_;

src/digital_output.h

@@ -82,7 +82,6 @@ class DigitalOutput {
     unsigned long last_triggered_;
     uint8_t trigger_duration_;
     uint8_t cv_pin_;
-    uint8_t led_pin_;
     bool on_;
 
     void update(uint8_t state) {