Complete the EnumFilterAttenuator device

[olliesilvester]

In #969, an EnumFilterAttenuator device was created, which is for attenuators where each filter has a selectable material rather than just being 'IN' or 'OUT'. I only created the filters as part of the previous PR, so we should create the rest of this device, using the EPICS interface of the vmxm or i24 attenuator device as a guide

Acceptance Criteria

• EnumFilterAttenuator contains all useful signals and logic