Create an Attenuator base class

[noemifrisina]

The current Attenuator device has a couple of PVs and filter specifications that do not match all beamlines, but most of it is common to most and could be reused. It would be useful to have one common attenuator base class with common signals that can be re-used to implement beamline specific devices.

Acceptance Criteria

• There is an Attenuator base class with common features across beamlines