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

[neworderofjamie]

This will require:

• A Pybind11 wrapper for plog::Record - all this needs to do is expose the getters (https://github.com/genn-team/genn/blob/master/include/genn/third_party/plog/Record.h#L221-L261) as read only properties
• A PyBind11 trampoline class (like https://github.com/genn-team/genn/blob/master/pygenn/src/genn.cc#L69), allowing the write method of plog::IAppender (https://github.com/genn-team/genn/blob/master/include/genn/third_party/plog/Appenders/IAppender.h#L13) to be overriden from Python
• A PyBind11 wrapper class for plog::IAppender.
• A Python-derived appender class which converts PLOG severity to Python logging severity and writes the message to the python logging system. We can create one of these at module-level in PyGeNN for each GeNN logging channel (https://github.com/genn-team/genn/blob/master/include/genn/genn/logging.h#L66-L70)
• Currently PLOG logging is initialised from Python by calling https://github.com/genn-team/genn/blob/master/pygenn/src/genn.cc#L258-L264. This can be deleted and the underlying GeNN::Logging::init function exposed directly.
• Similarly backend PLOG logging is initialised from Python by calling https://github.com/genn-team/genn/blob/master/pygenn/src/cudaBackend.cc#L35 (and equivalent from other backends). These should also be modified to take an appender

Wrapping virtual bases classes in PyBind11 is documented at https://pybind11.readthedocs.io/en/stable/advanced/classes.html#overriding-virtual-functions-in-python.