[Core] Refactor VMultiOpEntry

[alxbilger]

My goal was to simplify the writing of vector operations in ODE solvers.

Before

VMultiOp ops;
ops.resize(2);
// vel += dx * dt
ops[0].first = vel2;                                   // <-- What is first???
ops[0].second.push_back(std::make_pair(vel.id(),1.0)); // <-- What does a pair represent? What does this list represent?
ops[0].second.push_back(std::make_pair(dx.id(),dt));
// pos += vel * dt
ops[1].first = pos2;
ops[1].second.push_back(std::make_pair(pos.id(),1.0));
ops[1].second.push_back(std::make_pair(vel2.id(),dt));

to

VMultiOp ops(2);

// vel += dx * dt
ops[0] = VMultiOpEntry{ vel2,
    ScaledConstMultiVecId{vel.id(), 1._sreal} + ScaledConstMultiVecId{dx.id(), dt}};
// ok, now we know that the scalar and the MultiVecId are multiplied together.
// there is also an operator +, suggesting that the whole operation is a linear combination

// pos += vel * dt
ops[1] = VMultiOpEntry{ pos2,
    ScaledConstMultiVecId{pos.id(), 1._sreal} + ScaledConstMultiVecId{vel2.id(), dt}};

Note that in C++20, we can write something like:

ops[1] = VMultiOpEntry{ .outputId = pos2,
    ScaledConstMultiVecId{pos.id(), 1._sreal} + ScaledConstMultiVecId{vel2.id(), dt}};

which is IMO clearer.

NOT IMPLEMENTED:
I wanted to go even further (but there are problems of type conversion, so it's not yet ready):

VMultiOp ops(2);

// vel += dx * dt
ops[0] = VMultiOpEntry{ vel2, vel.id() + dx.id() * dt};

// pos += vel * dt
ops[1] = VMultiOpEntry{ pos2, pos.id() + vel2.id() * dt};

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