Add a way to limit properties serialized for agents

[LaCuneta]

Looking at the test case for Python, it's awesome we can serialize agents now. To think ahead a bit, the amount of data generated and serialized could be large with many agents. It might be handy to have a global command to set which fields are serialized by default. So if you only need xcor, ycor and who, then that is all you get (py:turtle-serials xcor ycor who).

[LaCuneta]

To add some clarity after further discussion: this idea is about optimizing the speed of interop and the amount of memory needed for it. Large numbers of agent variables that contain complex data (big lists, basically) across thousands of turtles would take a lot of time to serialize if it's done even per-tick only. It's not an immediate issue, but as people want to take advantage of the feature I can see this being a necessity later on.