assign_external_pointer overrides should use explicitly sized integer types

[neworderofjamie]

These are generated here:

https://github.com/genn-team/genn/blob/master/generate_swig_interfaces.py#L287-L292

But using unsized types makes numpy interfacing code irritating specifically as sizeof(long) is different on Windows and Linux. Correct solution would be using uint32_t etc. These would then need to be used here:

https://github.com/genn-team/genn/blob/master/pygenn/genn_model.py#L156-L172

[neworderofjamie]

Everything now uses sized types