ENH: add preliminary autosave-like features

[klauer]

Closes #13

• Simple pvproperty wrapper to mark fields/pvproperties as autosaved
• Appears to work for material, thickness, and is_stuck 👍

~Will be pushing this back to caproto as an example shortly.~ (see https://github.com/caproto/caproto/pull/617)

[klauer]

Proposed this to caproto upstream, but feedback is unlikely to happen any time soon, so proceeding here as this works well enough for the attenuator.