Start on making group_variable_set_conditional work -- currently subgroups are broken, though.
Add a read-only attribute for Variable objects
Add an `arange` function to devices that creates an `ArrayVariable` filled with numbers from 0 to size-1. This will be used for the variable "i" in a `NeuronGroup`.
(yet another) major overhaul of the indexing system, Synapses not quite working yet