GeNN 4.9.0

Release Notes for GeNN 4.9.0

This release adds a number of significant new features to GeNN as well as including a number of bug fixes that have been identified since the 4.8.1 release.
It is intended as the last release for GeNN 4.X.X.
Fixes for serious bugs may be backported if requested but, otherwise, development will be switching to GeNN 5.

User Side Changes

1. Implemented pygenn.GeNNModel.unload to manually unload GeNN models to improve control in scenarios such as parameter sweeping where multiple PyGeNN models need to be instantiated (#581).
2. Added Extra Global Parameter references to custom updates (see Defining Custom Updates, Defining your own custom update model and Extra Global Parameter references (#583).
3. Expose $(num_pre), $(num_post), $(num_batches) to all user code strings (#576)

Bug fixes

1. Fixed handling of indices specified as sequences types other than numpy arrays in pygenn.SynapseGroup.set_sparse_connections (#597).
2. Fixed bug in CUDA constant cache estimation bug which could cause nvLink errors in models with learning rules which required previous spike times (#589).
3. Fixed longstanding issue with setuptools that meant PyGeNN sometimes had to be built twice to obtain a functional version. Massive thanks to @erolm-a for contributing this fix (#591).

Optimisations

1. Reduced the number of layers and generally optimised Docker image. Massive thanks to @bdevans for his work on this (#601).