XIA PFCU4 filters and shutters

[canismarko]

I recently added support for the XIA PFCU4 filter bank to the haven library.

https://github.com/spc-group/haven/blob/main/src/haven/instrument/xia_pfcu.py

In our configuration, 2 of the 4 filters are actually shutters where one is the top half and the other the bottom half.

To start with I tried to make the shutter with the same interface as the ShutterBase found in apstools. This didn't work with pydm, though, as pydm needs a signal to set for open and close, and to read for its current state, etc., and it needs to device to be subscribable.

I started adding this capabilities to the PSS shutter class, and realized I was re-inventing the PVPositioner class, so I just made my shutter be a PVPositioner instead of an apstools ShutterBase, and used an enum instead of strings for the possible states so it would work with the PyDMByteIndicator widget.

My next step is to re-implement our pss shutter support using this positioner approach.

My question then is what, if anything, is of interest to apstools? I imagine the filter support minus the shutter would be useful. Would the shutter work be useful as well, with the caveat that it might not be backwards compatible?