Correct usage of the Eigen::Ref class

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In general, using a non const reference like Ref<T>& is never a good idea because this will only work if the caller already has a Ref<T> object at hand. This discards 5 and 7.

The cases 3 and 4 does not make sense, and they won’t compile in general.

Then, there is not much difference between a const Ref<const T> and a const Ref<const T>& because it is unlikely that the function is called with an existing Ref<const T> object, and so a Ref<const T> will have to be created in most cases anyway. Nevertheless, we could still recommend 1 over 2 or 6.

So in summary, we could recommend Ref<T> for a writable reference, and const Ref<const T>& for a const reference.

The option 6, Ref<const T>, might still be interesting if you want to change the matrix that is referenced (not its content) through a call to Ref constructor using placement new.

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