Support code should be usable by sparse connectivity initialisation

[jhnnsnk]

Several genn_model.create_custom_*_class() functions have the keyword argument support_code which I find very useful, for instance, for defining functions used by sim_code. However, this feature is currently not available for all functions of this type. In particular, I would appreciate if it could be added to genn_model.create_custom_sparse_connect_init_snippet_class().