控制节点连接删除后节点是否存在
当连接某个节点的其他节点都被删除时，MPxNode根据情况决定是否删除
控制此种删除行为的是以下几个函数：
| MStatus | setExistWithoutInConnections (bool flag) |
| This method specifies whether or not the node can exist without input connections. | |
| bool | existWithoutInConnections (MStatus *ReturnStatus=NULL) const |
| Determines whether or not this node can exist without input connections. | |
| MStatus | setExistWithoutOutConnections (bool flag) |
| This method specifies whether or not the node can exist without output connections. | |
| bool | existWithoutOutConnections (MStatus *ReturnStatus=NULL) const |
| Determines whether or not this node can exist without output connections. |
按照maya的规定，set函数应该在postConstructor调用，而不是在构造函数被调用。详细见postConstructor解释。
