REF: direct-to-PV vs signal protocol by way of cl

[klauer]

Description

Typhos supports two paths to get data from ophyd Signals.

1. Peeking into the EpicsSignal to see which PVs it is using and passing that along to PyDM
2. Directly through the ophyd subscription mechanism

This PR allows for an escape hatch for (1) in scenarios where we want typhos to use method (2).
This escape hatch comes in the form of a control layer cl check to see if the user has customized it beyond what ophyd supports.

Motivation and Context

Related to pcdshub/atef#17
Where we want archived data to be sourced rather than live data

How Has This Been Tested?

Interactively, so far

Where Has This Been Documented?

This PR text.

Screenshots (if appropriate):

This is a typhos screen using this PR that gets data from the EPICS archiver:

[ZLLentz]

This is much simpler than I thought it would be. I didn't notice that the control layers had name attributes.