Refactor controllers issue/200 & Test DepartmentController 255 (#266)

[manfredhod]

Description

Refactor of DepartmentController and unit tests added.

Fixes #200 & #255

Type of change

Delete unchecked boxes (only for Type of change)

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

The DepartmentController works as intended after refactoring. All the new unit tests pass.

Development Configuration Type "flutter --version" and "dart --version" in your CMD to check versions.

• Flutter version: 2.0.5
• Dart version: 2.12.3

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas, if necessary
• [x] I have made corresponding changes to the documentation, if necessary
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works, if necessary
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [ ] I have Acceptance Tested this on an iOS device
• [ ] I have Acceptance Tested this on an Android device