Extract L and U factors from IncompleteLUT

IncompleteCholesky has matrixL() that returns the L factor, but there is no equivalent operation for IncompleteLUT.

Consider adding

const FactorType& matrixL() const {
    eigen_assert(m_factorizationIsOk && "factorize() should be called first");
    return m_lu.triangularView<UnitLower>();
}

const FactorType& matrixU() const {
    eigen_assert(m_factorizationIsOk && "factorize() should be called first");
    return m_lu.triangularView<Upper>();
}

to the IncompleteLUT class?