Ability to unload PyGeNN models

[neworderofjamie]

PyGeNN GeNNModel objects wrap both a ModelSpec holding the model description and a SharedLibraryModel for loading the dynamic library created from the generated code and executing it. This PR lets you 'unload' the SharedLibraryModel component, freeing memory but allowing it to be easily re-loaded if required. This entails:

• In PyGeNN, resetting all views to None to prevent freed memory being accessed
• Adding a close method to SharedLibraryModel which sets all pointers to variables etc to nullptr, empties the cache used to speed up access to functions and unloads the shared library.

[tnowotny]

Ahem, sorry for the delay. This looks reasonable though I do not have the overview of whether everything is cleaned up that needs cleaning up. What would be a good test for this?

[neworderofjamie]

Will defer adding new tests until GeNN 5 when I reimplement feature tests in Python - raised #587 to remind me