MultiCubicSpline references an out-of-scope temporary if ae is defaulted in ctor

[pcaspers]

https://github.com/lballabio/QuantLib/blob/50ab670d72c3b64d07c7a8dd7f6a43582604f3f9/ql/math/interpolations/multicubicspline.hpp#L496

We might change the member to a non-reference type or change the ctor argument to optional<vector>?

[lballabio]

Yes, we can probably make that member a non-reference.

[pcaspers]

Yep, that's a good solution I think.

[github-actions[bot]]

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