Add pybind11 wrappings for Imath::Plane

[cary-ilm]

We are gradually working through replacing the existing Boost.Python wrappings of Imath classes with new wrappings using pybind11.

Follow the example of PyBindImath/PyBindImathBox.cpp as a template. Provide wrappings for all the methods covered in the original source, in the register_Plane function in PyImath/PyImathPlane.cpp.

Skip the array classes for now.

This requires some knowledge of pybind11 although it should be straightforward to follow the existing patterns.

This also requires a development environment with both Boost.Python and pybind11 installed.