Implement __get_item__ on Controller to access sub controllers

[GDYendell]

It is currently very verbose to access sub controllers

controller.get_sub_controllers()["R1"].voltage.disable_tracing()

Note this only one level deep.

Acceptance Criteria

This would become

controller["R1"].voltage.disable_tracing()

or a more extreme example for which I won't even write what is required currently

controller["FP"][0]["HDF"]["DS"].uid_datatype

[GDYendell]

@coretl is there any precedent for this in ophyd-async that we could follow / avoid?

[coretl]

For ophyd-async:

• Devices use attributes for child devices
• Unless they are DeviceVectors in which case they use int indexes
• This is because a Signal is a subclass of Device

So for your example the view from ophyd would be eiger.FP[0].HDF.DS.uid_datatype

[GDYendell]

OK, so that maps pretty well except that Attributes are not Controllers

[GDYendell]

I am going to try out copying the ophyd model and using . for sub controllers, as it seems nicer, but I think it will mean getting rid of root_attribute