deepSwap

[tianbingsz]

Fixes #566

With this PR, we can write code to directly deep swap the content of Vector and Matrix under the constraint of not allowing changing the data_ pointers.
Here is the example code to deep swap two Vectors,

para->getBuf(PARAMETER_SNAPSHOT_VALUE)->deepSwap(*para->getBuf(PARAMETER_VALUE))

and this is for the Matrices,

MatrixPtr cpuA = std::make_shared<CpuMatrix>(height, width);
MatrixPtr cpuB = std::make_shared<CpuMatrix>(height, width);
cpuA->deepSwap(*cpuB);

[wangkuiyi]

As here adds a new method, if we really think we need it, we'd write unit tests for it.

[tianbingsz]

done, passed deepSwap unit test test_matrixCompare

[wangkuiyi]

我们需要在comments里有更细致的描述，如何使用这个新增的method。我还没仔细研究如何自动从comments产生C++ API document的，可能可以问问 @hedaoyuan 。

[tianbingsz]

OK, will check with @hedaoyuan

[wangkuiyi]

We need to describe how are we going to use deepswap in this usage comment, and make sure that the example code comes into our auto-generated API documentation.

[tianbingsz]

done

[hedaoyuan]

I think it would be better to add a swap function that can support the exchange of data_ pointer. This should be more efficient than exchange all elements in the data_.

[wangkuiyi]

我没有问题了。但是 @hedaoyuan 有一个问题 。请 @tianbingsz 关注。

[tianbingsz]

@hedaoyuan， the use case is to swap the content of vector or matrix without changing the pointers.

[tianbingsz]

Just Sync with @hedaoyuan to clear the usage of this issue. Thanks for @hedaoyuan and @wangkuiyi insightful comments.