Ensure proper use of class variables

[DominicOram]

Fixes #810

Link to dodal PR (if required): DiamondLightSource/dodal#123

To test:

1. Confirm class variables are removed and tests still pass
2. Confirm exceptions are listed in the issue

[dperl-dls]

Nit: FGSComposite isn't really a singleton, we just only create it once, there's no enforcement for that. All the objects it holds are singletons though. Anyway, removing the class variables from FGSComposite doesn't change anything (they are all overwritten with instance vars on init) so you might as well remove those too if you feel like it

[DominicOram]

Nit: FGSComposite isn't really a singleton, we just only create it once, there's no enforcement for that. All the objects it holds are singletons though. Anyway, removing the class variables from FGSComposite doesn't change anything (they are all overwritten with instance vars on init) so you might as well remove those too if you feel like it

Yh, I think I meant that all the devices under it are singletons anyway so it's effectively a singleton. But I've changed it now anyway, thanks.

Tests are failing due to #847