Use GeNN's initvar snippets for synapse creation

[tnowotny]

GeNN supports in-parallel initialisation of synaptic connectivity matrices using "initvar snippets" of code if the synaptic connectivity is defined algorithmically (http://genn-team.github.io/genn/documentation/4/html/d4/dc6/sectVariableInitialisation.html). This would be more efficient in such cases than the current Brian2GeNN solution of C++ based initialisation inherited from Brian 2 cpp_standalone.