Use std::vector::reserve in Element::Clone

[scpeters]

🦟 Bug fix

Part of #1478

Summary

The performance of Element::Clone is poor, as noted in #1478. This is a small change to attempt to improve memory allocation.

There are several for-loops in Element::Clone that call std::vector::push_back, and since the vector size is known in advance, use std::vector::reserve to avoid multiple vector resizes.

I ran the unit tests with this change, and I didn't observe any noticeable speed-up.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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