reshape() not in tensor namespace (cmake-branch)

[wavepacket]

I just noted a smaller issue in the CMake branch of the tensor library.

The function reshape(Tensor<>, Dimension) is defined inside the Tensor class with an inline friend definition. According to the C++ specification, such a construct creates a free function that is a friend of the Tensor class. At least g++ creates this friend function outside of the tensor namespace; I have not checked the small print if this is standard behavior or a defect, but it is definitely an issue.

In principle, this should rarely be a problem: Most people should be "using namespace tensor" . However, it is surprising behavior that tensor::reshape() does not resolve to an existing function anymore.

[wavepacket]

The same issue also affects the function flatten().