Collision groups let you specify an integral group index. You can have all fixtures with the same group index always collide (positive index) or never collide (negative index) it have high priority than bit masksPhysicsBody的Group1.优先级比bit masks的优先级高(CategoryBitmask,ContactTestBitmask,CollisionBitmask)2.group是正数且相等的话，就一定碰撞3.group是负数且相等的话，就一定不会碰撞4.group其他情况没说，应该是取决于bit masks了
void setCategoryBitmask(int bitmask);
A mask that defines which categories this physics body belongs to. Every physics body in a scene can be assigned to up to 32 different categories, each corresponding to a bit in the bit mask. You define the mask values used in your game. In conjunction with the collisionBitMask and contactTestBitMask properties, you define which physics bodies interact with each other and when your game is notified of these interactions. The default value is 0xFFFFFFFF (all bits set).PhysicsBody的CategoryBitmask1.CategoryBitmask用来标记这个PhysicsBody的所属类别2.每个PhysicsBody最多可以属于32个类别，一个bit代表一个类别，0代表不是对应类别，1代表是对应类别3.和另外两个属性(collisionBitMask and contactTestBitMask)配合着用4.默认值0xFFFFFFFF，即这个PhysicsBody分别属于所有的32个类别
void setContactTestBitmask(int bitmask);
A mask that defines which categories of bodies cause intersection notifications with this physics body. When two bodies share the same space, each body’s category mask is tested against the other body’s contact mask by performing a logical AND operation. If either comparison results in a non-zero value, an PhysicsContact object is created and passed to the physics world’s delegate. For best performance, only set bits in the contacts mask for interactions you are interested in. The default value is 0x00000000 (all bits cleared).PhysicsBody的ContactTestBitmask1.ContactTestBitmask用来定义哪个类别的PhysicsBody可以和这个PhysicsBody产生相交通知2.举个例子：有PhysicsBody A B占了同样的空间了。如何判断A B要产生相交通知呢，if((A.CategoryBitmask & B.ContactTestBitmask) != 0 || (A.ContactTestBitmask & B.CategoryBitmask) != 0) {/*条件成立，执行相交通知*/}else{/*没有相交通知*/}3.为了性能考虑，相交通知只应该设置给我们感兴趣的物体4.默认值0x00000000 (all bits cleared)，没有哪个类别的PhysicsBody可以产生相交通知
void setCollisionBitmask(int bitmask);
A mask that defines which categories of physics bodies can collide with this physics body. When two physics bodies contact each other, a collision may occur. This body’s collision mask is compared to the other body’s category mask by performing a logical AND operation. If the result is a non-zero value, then this body is affected by the collision. Each body independently chooses whether it wants to be affected by the other body. For example, you might use this to avoid collision calculations that would make negligible changes to a body’s velocity. The default value is 0xFFFFFFFF (all bits set).PhysicsBody的CollisionBitmask1.CollisionBitmask是用来定义哪个类别的PhysicsBody可以和这个PhysicsBody产生碰撞2.举个例子：有PhysicsBody A B彼此接触了，那碰撞就可能发生。A受不受这个碰撞的影响(体现为物理效果)表达为：if((A.CollisionBitmask & B.CategoryBitmask) != 0 ){/*A产生碰撞的物理效果*/} else{/*A没效果*/}3.至于B受不受这个碰撞的影响，则采用同样的表达式单独计算即可if((B.CollisionBitmask & A.CategoryBitmask) != 0 ){/*B产生碰撞的物理效果*/} else{/*B没效果*/}4.默认值0xFFFFFFFF (all bits set)，所有类别的PhysicsBody都能对this body产生碰撞效果
a b为同一类物品，group都取1。有碰撞现象，无回调【跟group不设置时现象一样，还不能说明什么】
a b为同一类物品，group都取-1。无碰撞现象，无回调，物体a b分别从同一个位置落下后，a b重叠【其他bitmask的默认值设置是有碰撞现象的，但设置group取相同的负数后没碰撞了，group优先级高！】
a b为同一类物品，group都取1，但设置CollisionBitmask=0x00(试图不要碰撞效果)。现象1自由落体到世界外面去了，现象2快速touch在同一个位置产生两个精灵会有挤开的现象(若group取-1则不会挤开，叠在一起了)。【说明group的优先级最高！】
a b为同一类物品，group都取1，但设置CollisionBitmask=0x00(试图不要碰撞效果)，设置ContactTestBitmask=0x01(试图获取回调)。现象1自由落体到世界外面去了；现象2快速touch在同一个位置产生两个精灵会有挤开的现象(若group取-1则不会挤开，叠在一起了)；现象3有回调。【说明group的优先级最高！】
想要回调
属性值：group =0CategoryBitmask =0xFFFFFFFFContactTestBitmask=0x00000001<-----change to thisCollisionBitmask =0xFFFFFFFF
