EPICS Record support: common structures

Some fields are common [1] to all [2] or some EPICS records. These are defined as mixin classes for the various ophyd-style devices.

[1]

https://docs.epics-controls.org/en/latest/guides/EPICS_Intro.html#ioc-database,

[2]

https://epics.anl.gov/base/R7-0/6-docs/RecordReference.html