Vec3: add a double-precision constructor

src/main/java/com/github/stephengold/joltjni/Vec3.java

@@ -56,6 +56,19 @@ public Vec3() {
         this.z = 0f;
     }
 
+    /**
+     * Instantiate a vector with specified components.
+     *
+     * @param x the desired X component
+     * @param y the desired Y component
+     * @param z the desired Z component
+     */
+    public Vec3(double x, double y, double z) {
+        this.x = (float) x;
+        this.y = (float) y;
+        this.z = (float) z;
+    }
+
     /**
      * Instantiate a vector with specified components.
      *