Unused object warning

[mstimberg]

This addresses the issue in #1499, by raising a warning if an object gets garbage collected but has never been included in any Network. Instead of dealing with the complexities of weak references and the InstanceTracker, as initially proposed in #1499, this simply uses the __del__ function in BrianObject. This seems to be working well, and running it against our examples I did not encounter any issues. There are false positives for "artificial" examples like https://brian2.readthedocs.io/en/stable/examples/synapses.efficient_gaussian_connectivity.html, since they create Synapses but do not ever run a simulation. In principle, we could have another check that only triggers these warnings when there has been a run at some point, but I am not sure whether it is worth the complication.
This code from the forum (where the define_network function only returns the NeuronGroups, but not the Synapses) is a good example for the kind of code that will trigger a warning: https://brian.discourse.group/t/synapses-not-functioning/1118

[thesamovar]

This seems fine to me, although I have to admit I didn't entirely understand the __del__ logic.

[mstimberg]

This seems fine to me, although I have to admit I didn't entirely understand the __del__ logic.

The reasoning is: objects fill in their _network attribute when they are simulated as part of a Network – this is used for the errors when an object has been simulated as part of different networks. We therefore raise the warning when an object gets garbage collected when an object has its attribute _network still as None (for some reason it can happen that the attribute does not exist at all, but we can simply ignore that).

The second check is to avoid that objects such as StateUpdater, Resetter, etc. also raise this warning. It's a bit of a hack, we simply assume that everything that has a group attribute is something dependent on other objects and not worth warning about. The other alternative would be to overwrite the __del__ method in all the subclasses. We actually need to do this for Subgroup, because it stores its reference to the main group as source, not as group 🙃