(cmake branch) Views cannot be reshaped

[wavepacket]

As the title suggests, it is not possible to directly reshape a tensor slice / view, that is,

CTensor x = CTensor::random(2, 3, 4);
CTensor y = reshape( x(_, 3, 4), 12);

fails to compile.
The workaround consists of assigning the slice to a new tensor, then reshaping this tensor. This workaround should not even carry significant performance overhead.