Moved pybind11 Euler to src/pybind11

[faizhaque]

Moved the file to the correct file location.

[cary-ilm]

Thank you, this looks better! There are still a few missing bits that hook the new methods into the configuration.

Look at PR #440, which does the same thing for the Imath::Plane and Imath::Line objects, try to follow that pattern:

1. Added the file to CMakeLists.txt so it gets compiled.
2. Add the call to register_imath_euler(m) to pybindimathmodule.cpp (and the forward declaration to `PyBindImath.h')

Thanks!