Have a consistent way to set/get parameters for all objects in PyNN

[apdavison]

To set parameters on objects in PyNN, some objects have obj.set(param=val), others obj.set_parameters(param=val) or obj.param = val.

This difference is often confusing (see #380, #381, #418). We should consider choosing one of these, and implementing it for all relevant classes, with deprecation warnings for the old-style.

The downside of this is that it might break a lot of existing code.