Interpret the dtype keyword argument in a consistent way between `NeuronGroup` and `Synapses` by introducing a helper function `dtype_dictionary` in `Group`
Make `StateMonitor` into a `Group` (not working in standalone yet).
Change handling of `AttributeVariables` in standalone and make special handling of t and dt no longer necessary. The variable `N` is no longer automatically provided to templates so it has now to be included in `USES_VARIABLES`. Monitors now work again with standalone.
Do not show warnings about internally used variables, i.e. variables used in templates or specified via `needed_variables`. Otherwise a user would get a warning whenever `N` is defined in a script, even if it is never used in string expressions.