Redirect log output from C++ to Python

[neworderofjamie]

Currently, messages written to GeNN log channels (e.g. our nice new transpiler error messages) go to stdout which is often hidden in Python e.g. in notebooks. On Linux we could just redirect these using e.g. https://pypi.org/project/wurlitzer/ but that doesn't work on Windows. Wrapping plog::IAppender with pybind11 would probably be the most elegant solution