Proof of concept part of cpython extension using Pybind11

[sloretz]

Like #172 this is a proof of concept meant to generate discussion. It implements a WaitSet class in c++ and uses pybind11 to create a cpython extension. To build first pip3 install pybind11.

Advantages over Cython

• No new language to learn: Pybind11 is pure C++, cython uses a superset of python
• c++ Linter tests work, cython files don't have a linter
• Do not need to maintain duplicate definitions for C methods (compare with rcl.pxd)
• No code generation step: Pybind11 uses C++ templates that expand to python extension code at compile time. Cython has to generate C code that is then built.
• Enables rclcpp and rclpy to possibly resuse C++ code.
• pybind11 is a "lighter" dependency since it does not install any executables

Disadvantages compared to Cython

• Cython is in debian
• Cython PR Proof of concept part of cpython extension using Cython #172 required slightly less code (rcl.pxd + _rclpy_wait_set.px = 208 lines, _rcl_wait_set.cpp = 244)
• When calling a cpython extension API directly code using pybind11 still needs to check if a python exception was raised. Cython generated does this automatically (compare call to PyCapsule_GetPointer here with call using cython).
  • Not as big of a deal as it seems, being able to throw a C++ exception that gets translated to a python exception makes error handling much easier than the current state.
  • This PR uses is_subscription_ready instead of ready_subscriptions and pysubs is not actually the python type because it would have been a little more verbose with the checks.
• Default parameter values are separated from C++ function definition (see default value of timeout on wait here and when using cython)

Same as Cython

• Both reduce the code needed to check for python exceptions
• Both are in homebrew
• Both limited to interpreters that implement cpython's extension API
• Both can be rolled out in incrementally: no need to rewrite all of the C code in rclpy right away.
• Both can be built using CMake or a setup.py
• Both are only build dependencies, the user doesn't need anything new installed at run time

Misc info

• Requires Visual Studio 2015 or higher
• Released under a BSD license
• Pybind11 is a header only library
• Pip installation of pybind11 does not come with a CMake <project>Config.cmake. It appears to have some nice CMake code if it is installed from source.

connects to #172

[sloretz]

This is pretty stale, and just a proof of concept, so I'll close it.