These headers, providing routines to construct and navigate bounding volume hierarchies, have been copied from https://github.com/madmann91/bvh commit 66e445b92f68801a6dd8ef94.

Minor changes have been subsequently been applied to achieve compilation with C++17:

inclusion of alternative span, when std::span is not found
replacement of C++20 defaulted comparison operators with actual implementation
old-style struct construction for objects of type "Reinsertion"
use of std::inner_product instead of std::transform_reduce (gcc 8.5 had problems)

This is needed since ROOT should compile with C++17.