Function getDiagonals

tests/utils/qmatrix.cpp:246–255

 */

qvector getDiagonals(qmatrix m) {
    DEMAND( m.size() > 1 );

    qvector out = getZeroVector(m.size());

    for (size_t i=0; i<out.size(); i++)
        out[i] = m[i][i];
    
    return out;
}