Add a for loop that is unrolled at compile time

[maxpkatz]

Summary

The constexpr_for function is fully unrolled at compile time. This is useful for relatively short loops where some of the functions inside the loop are known to possible to evaluate at compile time and may be relatively expensive, so evaluating them at compile time rather than at runtime may be beneficial for performance reasons.

Additional background

This has been used in AMReX-Astro/Microphysics successfully in the context of evaluating some nuclear reaction network quantities at compile time.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

I am not sure how what this comment about [&] means. If the loop body uses a pointer (a raw pointer or Array4), I don't think you need [&] and [=] would also work. (I am not saying = is better.) (Edited to add not in my last sentence. I unfortunately have the tendency of having "not" in my mind and not typing it.)

[maxpkatz]

That was there because of its specific usage in Microphysics. You are right that it is not helpful when copied to AMReX for general usage. I've removed that comment just to avoid confusion.