Fixed some issues with sparse array indexing, especially in the CPU code. The previous examples are all functional, but learning with sparse synapses does not work.
Also modified the MBody_userdef to have learning synapses with sparse connectivity, therefore it is expected to have a wrong behaviour as it is now.
Merge branch 'development' of https://github.com/genn-team/genn into development

Conflicts:
	lib/src/generateRunner.cc