Prototype nanobind bindings for Imath

[cary-ilm]

We have the long term goal of removing the dependency on Boost.Python, and the current strategy is to rewrite the bindings with pybind11, although nanobind promises to be even better than pybind11, and also claims to be largely API-compatible.

We should prototype bindings using nanobind. This would involve duplicating the current state of the pybind11 bindings, both source and CMake configuration, and replacing both to use nanobind.

[mnxn]

I'll take a shot at implementing this for Dev Days next week.