Add Angle unit and remove unused code from Length unit

lib/src/angle.dart

@@ -0,0 +1,31 @@
+import 'dart:math';
+
+class Angle implements Comparable<Angle> {
+    /*
+   * The value of this Angle object in degrees.
+   */
+    final double _angle;
+
+    ///degrees to radians
+    static const double degree_to_rad = PI / 180.0;
+
+    ///radians to degrees
+    static const double rad_to_degree = 180.0 / PI;
+
+    Angle.fromDegrees({double value : 0.0}): _angle = value;
+    Angle.fromRadians({double value: 0.0}): _angle = value * rad_to_degree;
+
+    double get inDegrees => _angle;
+    double get inRadians => _angle * degree_to_rad;
+
+    /**
+     * Compares this Angle to [other], returning zero if the values are equal.
+     *
+     * Returns a negative integer if this `Angle` is shorter than
+     * [other], or a positive integer if it is longer.
+     *
+     * A negative `Angle` is always considered shorter than a positive one.
+     *
+     */
+    int compareTo(Angle other) => _angle.compareTo(other._angle);
+}

lib/src/length.dart

@@ -1,9 +1,4 @@
-import 'dart:math';
-
 class Length implements Comparable<Length> {
-    /// 1 meter
-    static const double _one_meter = 0.999999690624;
-
     ///meters to kilometers: meters * 10^-3
     static const double _m_to_km = 0.001;
     /// meters to miles: meters * 6.21371192 * 10^-4