Added new VCN_OpenLoop device class

[jozamudi]

Description

Added VCN_OpenLoop device class to pcdsdevices valve.py. VCN_OpenLoop uses VCN as a template w/ the removal of 'open' and 'position_readback' commands. The 'state' member vairable has been renamed to 'control_mode' and the associated doc string was been updated.

Motivation and Context

https://jira.slac.stanford.edu/browse/ECSENG-848

How Has This Been Tested?

Used Typhos to check if device would load correctly.

Where Has This Been Documented?

Pre-merge checklist

• [x] Code works interactively
• [x] Code contains descriptive docstrings, including context and API
• [x] New/changed functions and methods are covered in the test suite where possible
• [x] Test suite passes locally
• [x] Test suite passes on GitHub Actions
• [x] Ran docs/pre-release-notes.sh and created a pre-release documentation page
• [x] Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

Other than my docs note I see no problems here- I'd like to let Jing's approval gate the PR though, she has necessary context that I do not have.