Preserve Directory Structure for NanoVDB util/ and cuda/ Headers on Installation

[eclipse0922]

This PR addresses an issue where the directory structure of the util/ and util/cuda/ headers in NanoVDB is not preserved when installed through CMake. The current installation process flattens the directory structure, leading to incorrect include paths when NanoVDB is used from the installed directories.

For example, https://github.com/AcademySoftwareFoundation/openvdb/blob/master/nanovdb/nanovdb/util/GridHandle.h uses cuda utils like this, but there is no 'cuda' directory in installed path.

#if defined(__CUDACC__)
#include <nanovdb/util/cuda/CudaGridHandle.cuh>
#endif// defined(__CUDACC__)

#endif // NANOVDB_GRID_HANDLE_H_HAS_BEEN_INCLUDED

Changes Modified the CMakeLists.txt to use install(DIRECTORY ...) instead of install(FILES ...) for the util/ directory. This change ensures that the directory hierarchy (specifically the util/cuda/ subdirectory) is maintained during installation.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: eclipse0922 / name: sewon jeon (4d3be1aed44a7f2449666cd358bc7da38f7a1279, e136eeeb277fd5a45f5e1ef92d37d0fcdac76913)

[eclipse0922]

Another approach: Split NANOVDB_INCLUDE_UTILFILES into two parts, cuda and non-cuda, then install them separately.