Fix/followup jf precorrection

Follow up for discussions on https://git.xfel.eu/detectors/pycalibration/-/issues/63

Description

1. Remove try/except for memory cells keyError
2. Remove Overwrite.
3. Small mentioned refactors.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• Refactor (refactoring code with no functionality changes)

Checklist:

Reviewers

@kluyvert @schmidtp