Added Vector.subtract that takes individual components.

Closes #11.

src/main/java/pythagoras/f/AbstractVector.java

@@ -136,6 +136,16 @@ public abstract class AbstractVector implements IVector
         return result.set(x() + x, y() + y);
     }
 
+    @Override // from interface IVector
+    public Vector subtract (float x, float y) {
+        return subtract(x, y, new Vector());
+    }
+
+    @Override // from interface IVector
+    public Vector subtract (float x, float y, Vector result) {
+        return result.set(x() - x, y() - y);
+    }
+
     @Override // from interface IVector
     public Vector addScaled (IVector other, float v) {
         return addScaled(other, v, new Vector());

src/main/java/pythagoras/f/IVector.java

@@ -115,6 +115,14 @@ public interface IVector
      * @return a reference to the result, for chaining. */
     Vector subtract (IVector other, Vector result);
 
+    /** Subtracts a vector from this one.
+     * @return a new vector containing the result. */
+    Vector subtract (float x, float y);
+
+    /** Subtracts a vector from this one and places the result in the supplied object.
+     * @return a reference to the result, for chaining. */
+    Vector subtract (float x, float y, Vector result);
+
     /** Rotates this vector by the specified angle.
      * @return a new vector containing the result. */
     Vector rotate (float angle);