Let second_derivative() return a const reference, rather than a value.
Let second_derivative() return a const reference, rather than a value.

This function is used in places where we want to get pointers into an array. Make sure that
is possible for both the const and the non-const version of the function. (The latter
already did this; the former didn't and that wasn't needed, but I need it for a
follow-up patch.)

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