[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.

Reviewers

@ahmedk