Avoiding dev/prod conflicts of device definitions

[klauer]

Issue

Per @ZLLentz:

I wonder if there's any way to avoid this problem:

1. A new class is added to pcdsdevices
2. happi is updated to expect the new class
3. All old code that referenced this device needs a pcdsdevices update or the load fails

It makes me wonder if we need to do versioning on these or have some other way to maintain backward compatibility in this sort of environment...

Originally posted by @ZLLentz in #41 (comment)

Possibilities

Our thoughts for avoiding this:

1. Device-specific classes as in AT1K4 with helpful docstrings (@klauer)
2. Factory methods for creating devices (@tangkong)