Speed up general transpose

[nstair]

Speed/clean up the general_transpose function (in tensor.cc). Currently the last for loop in

Tensor Tensor::general_transpose(const std::vector<size_t>& axes) const 
{
    if (axes.size() != ndim()) {
        throw std::invalid_argument("Invalid axes permutation");
    }

    std::vector<size_t> transposed_shape(ndim());
    for (size_t i = 0; i < ndim(); ++i) {
        transposed_shape[i] = shape_[axes[i]];
    }

    // std::shared_ptr<Tensor> transposed_tensor(new Tensor(transposed_shape));
    Tensor transposed_tensor(transposed_shape);

    std::complex<double>* transposed_data = transposed_tensor.data().data();
    // const std::complex<double>* original_data = data_.data();

    // This works but probably can be made more efficient.
    // Fix if it turns out to be a bottleneck
    for (size_t i = 0; i < size_; i++){
        std::vector<size_t> tidx_trans = vidx_to_tidx(i);
        size_t t_vidx = transposed_tensor.tidx_to_trans_vidx(tidx_trans, axes);
        transposed_data[t_vidx] = data_[i];
    }

    return transposed_tensor;  
}

is maybe doing too much work, if you have time and see a way to optimize then give it a try. Try to transpose a large tensor (30x30x30x30) or someting and see if it is faster.