add magnitude function in Vector class (#34)

Add `magnitude` function in `Vector` class

Author: battlecook

src/Vector.php

@@ -74,6 +74,14 @@ public function length(): float
         return $this->l2norm();
     }
 
+    /**
+     * @return float
+     */
+    public function magnitude()
+    {
+        return $this->length();
+    }
+
     /**
      * @param self $other
      * @return float