Scan shifting

• Rebase
• Added functionality
• Added tests

While working on scanning, I noticed that I was checking the validity of corner markers multiple times in the processing of a scan page. To fix this, I seperated the functions a bit so I could add a validity check right after detecting corner markers. The current workflow is now:

1. Decode data matrix (not added here but in MR !24 (merged) )
2. Detect corner markers
3. Check validity of corner markers
4. Rotate scan
5. Shift scan
6. Detect value of ID grid