[WIP] Feature: Add Python bindings

[wbthomason]

This PR adds a generator for Python bindings, using pybind11 and libclang. It is intended as a MVP for Python bindings; additional features (e.g. bidirectional bindings to allow overriding of virtual methods from Python) would be good to add in the future.

This isn't 100% ready for merge yet, but it's done enough that I wanted to make a draft PR to start getting eyes on the work. Currently, the generate_bindings.py script can successfully generate pybind11 code for the entirety of robowflex_library, with the following caveats/TODOs which should be resolved prior to merge.

• It doesn't currently recognize and filter out template methods (these are only supported in specific instantiations)
• No support for keyword/default arguments
• generate_bindings could use some cleanup and commenting/documentation
• We don't yet have a solution for generating bindings for external types that robowflex needs, e.g. moveit_msgs::RobotState (which currently blocks fetch_test.py from executing correctly)
• Integration with the build isn't quite right: the extension library isn't installed to Python's search path
• [optional] Should allow multiple modules/a hierarchical structure
• [optional] Should allow combination with manual bindings for special cases

[wbthomason]

Should consider switching to https://github.com/wjakob/nanobind - it's pybind11 (by the same author), but for C++17 and faster (compile time and run time) as a result.