add author info

src/BulletSoftBody/btPreconditioner.h

@@ -1,4 +1,6 @@
 /*
+ Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
+ 
  Bullet Continuous Collision Detection and Physics Library
  Copyright (c) 2019 Google Inc. http://bulletphysics.org
  This software is provided 'as-is', without any express or implied warranty.
@@ -18,7 +20,6 @@ class Preconditioner
 {
 public:
     typedef btAlignedObjectArray<btVector3> TVStack;
-//    using TVStack = btAlignedObjectArray<btVector3>;
     virtual void operator()(const TVStack& x, TVStack& b) = 0;
     virtual void reinitialize(bool nodeUpdated) = 0;
 };