Update tracker parameter in simulation.py

[galenseilis]

Default argument was a mutable type. Generally the advice I've read is to only use immutable types as defaults. Although I was not running into this issue, it can in principle lead to unusual behavior when a default argument is mutable.

In this case I have changed the default argument of tracker to None, but when this happens the same tracker will be assigned as before.

[geraintpalmer]

Thank you, I didn't know about this. Sounds like a good change :+1: