Ensure we're not holding state in class variables

[DominicOram]

See issue fixed in b9d46d5. We should go through the code and ensure we're not doing the same elsewhere, this is particularly a problem where we're e.g. appending but we should fix it more generally too

[DominicOram]

Going through the classes:

• For dataclasses and pydantic models this isn't an issue, instance variables will be created to override the class variables
• BlueskyRunner is a singleton so it's fair to keep class variables
• DCGIDFilter is a singleton
• FGSComposite is a singleton
• ophyd Device classes handle this with the Component mechanism