[AGIPD] Modernise use of extra_data to get AGIPD control data
Description
This streamlines the code to look up AGIPD control properties a bit. It shouldn't change any functionality. Not a big deal, but I figure that code we maintain may as well illustrate how EXtra-data can be used elegantly. 
How Has This Been Tested?
TBD in CI
Types of changes
Checklist:
- My code follows the code style of this project.
