Update nanort version and include path.

[jdumas]

This PR updates the include path of nanort in geometry central to facilitate using the upstream nanort repository. I.e. users should #include <nanort.h> and not <nanort/nanort.h>.

Additionally, since nanort in geometry central is 8 years old, I though we could update it as well, as geometry central code isn't compatible with the latest nanort API.

[nmwsharp]

Thank you for submitting this. I believe this is fully handled by the changes in #242 incorporating these. If not please re-open!

There were several overlapping issues/PRs about the way nanoflann/nanort are handled.

I tried to unify & fix so now:

• Both nanort and nanoflann live on their standard paths
• Remove old backwards compatible code preserving old paths (this is a small breaking change, better than the confusion of multiple paths)
• Make nanort an private dependency similar to nanoflann make install/link in large codebases easier
• Create a small tracer class exposing nanort

[jdumas]

Thanks. Out of curiosity, have you thought about using FetchContent/CPM to manage those dependencies? Then you could setup something like Dependabot/Renovate to keep them updated without breaking a sweat. It should be less work in the long run, and I'm happy to help if you want.