Move away from std functions inside Packs implementation

[bartgol]

These cause plenty of false positive warnings on GPU, and are a bit annoying. Instead, we should completely rely on stuff in Kokkos_MathematicalFunctions.hpp. Namely, I think we should

• Use Kokkos::xyz instead of std::xyz inside the pack loop of the fcn overload
• Inject our pack utilities into the Kokkos namespace, so that downstream templated code can do Kokkos::sqrt(my_value) without having to worry whether my_value is a builtin type or a pack.
• Remove ekat::impl::max and ekat::impl::min, in favor of the ones provided by Kokkos.