Add structured binding support to GpuTuple

[AlexanderSinn]

Summary

This PR adds C++17 structured binding support to amrex::GpuTuple.

Additional background

See case 2 in https://en.cppreference.com/w/cpp/language/structured_binding.
Note that structured bindings will also use the existing amrex::get.

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]

Nice! It works as expected. Note that since specializing std::get (a function template) has been outlawed since C++20, but specializing std class templates are still legit.