Add a `set` to the AttenuatorBase device

Spawend from #889

Right now the `AttenuatorBase` device is only used for reading but there are a couple of PVs that can be set in there (comment reported here):

> Either this attenuator is only used for reading and we remove these PVs or we want to add the setting too, in which case AttenuatorBase should have a set.
>
> If we're going to do setting as well I actually think you might be able to get away with exactly the same set between the two. In the i24 case:
>
> _filters_in_position contains only BL24I-OP-ATTN-01:MP1:INPOS and BL24I-OP-ATTN-01:MP2:INPOS
> _calculated_filter_states are soft signals that are all high
> _change is a soft signal that's a no-op

## Acceptance Criteria
* There is a set in the AttenuatorBase device