Expression templates

[gf712]

Like in Eigen, we should have an optimisation with expression templates. Right now the library creates temporary matrices/vectors for each operation. The paper says that we are allowed to return what we think is best from the four overloaded operators (+,- (negation and subtraction) and *) defined in arithmetic_operators.hpp. My idea is to return an expression template, eigen style, for example addition:

template<class OT, class ET1, class OT1, class ET2, class OT2>
struct matrix_addition_traits<OT, matrix<ET1, OT1>, matrix<ET2, OT2>>
{
    // not sure yet how these lines would have to change
    using engine_type = matrix_addition_engine_t<OT, ET1, ET2>;
    using op_traits   = OT;
    using result_type = matrix<engine_type, op_traits>;

    matrix_addition_traits(matrix<ET1, OT1> const& m1, matrix<ET2, OT2> const& m2): lhs(m1), rhs(m2) {}
    
    // do the actual calculation
    auto     operator ()(typename result_type::index_type i, typename result_type::index_type j)
    {
         return lhs(i, j) + rhs(i, j);
    }

    // hold reference to lhs and rhs matrices
    const matrix<ET1, OT1>& lhs;
    const matrix<ET2, OT2>& rhs; 
    
    // syntatic sugar that calls the constructor (rather than calculate the value explicitly with allocation)
    static auto  add(matrix<ET1, OT1> const& m1, matrix<ET2, OT2> const& m2);
};

would be good to discuss a bit how we could achieve this and remain compliant with P1385. It seems like they used Eigen quite a bit as a de#spiration, so this should be doable.