Develop standard method of easily creating Prefs

[blu28]

Often we have constants in the Constants.java file that are tunable. Ideally, we should have a way to change them if circumstances warrant it. We would still like to assign them to the same constant field names and have them be static final. We should also log the values to the telemetry. But allow them to be changed via the prefs interface. They would be read once at robot startup (changing them would require a reboot of the software).

[blu28]

See also issue #33

[blu28]

See also issue #70

[blu28]

A first cut at doing this would define the key and backup values in the Constants.java file, then create a set of initPreferences and loadPreferences methods to initialize them and load them using the respective "init" and "get" methods from the Preferences class. This is a little ugly since the programmer needs to know the name of the constants for the key and back up.

I think there might be a way to do this with all the code in the Constants file. Use enum classes to set the values for the key and back up and the enum name with the name we would have used for it if it were a constant.

This example enum class should give some clues:

public enum Planet {
    MERCURY (3.303e+23, 2.4397e6),
    VENUS   (4.869e+24, 6.0518e6),
    EARTH   (5.976e+24, 6.37814e6),
    MARS    (6.421e+23, 3.3972e6),
    JUPITER (1.9e+27,   7.1492e7),
    SATURN  (5.688e+26, 6.0268e7),
    URANUS  (8.686e+25, 2.5559e7),
    NEPTUNE (1.024e+26, 2.4746e7);

    private final double mass;   // in kilograms
    private final double radius; // in meters
    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }
    private double mass() { return mass; }
    private double radius() { return radius; }

    // universal gravitational constant  (m3 kg-1 s-2)
    public static final double G = 6.67300E-11;

    double surfaceGravity() {
        return G * mass / (radius * radius);
    }
    double surfaceWeight(double otherMass) {
        return otherMass * surfaceGravity();
    }
}