Extremely bug: get_DMK_vector()!

[dyzheng]

Describe the bug

template <typename TK, typename TR>
std::vector<std::vector<TK>> DensityMatrix<TK, TR>::get_DMK_vector() const
{
    return this->_DMK;
}

Expected behavior

The return should be a reference!
Please fix it ASAP.

To Reproduce

No response

Environment

No response

Additional Context

No response

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