FR: `model_base` methods - use `Eigen::Ref<Eigen::VectorXd>`

[andrjohns]

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Summary:

The Eigen overloads for model base methods like log_prob and unconstrain_array only accept Eigen::VectorXd inputs, which can require some data-reshaping/copying for downstream interfaces.

If these could be changed to Eigen::Ref<Eigen::VectorXd> (or overloads for these added), then the current Stan behaviour would be unchanged but interfaces could then pass Eigen::Map<Eigen::VectorXd> types and avoid extra copies

Current Version:

v2.35.0