Add documentation, cleanup

app/include/sensor/Sensor.h

@@ -0,0 +1,15 @@
+#ifndef SENSOR_H_
+#define SENSOR_H_
+
+#include <iostream>
+
+class Sensor {
+public:
+  // Read the current value
+  virtual int value() = 0;
+  // Name for displaying. Should be descriptive, e.g. "GPU" or the label from
+  // libsensors.
+  virtual std::string name() = 0;
+};
+
+#endif // SENSOR_H_