[AGIPD][PC][FF][PT2] Modifications to the plotting and fitting for both Slopes notebooks.

Description

This MR consists of changes by @jsztuk after updating plottings for PC and FF AGIPD notebooks. This branch is based on https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/444

• Fixes for the plotting functions.
• Fitting parameter modifications for the FF notebook

How Has This Been Tested?

This was tested by @jsztuk . Are there reports to attach here?

PC:

run_calibrate.sh

FF:

run_calibrate.sh

Relevant Documents (optional)

FF: GainCharacterization.pdf

PC: CharacterizeAGIPDPulseCapacitorData.pdf

Types of changes

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

Checklist:

• My code follows the code style of this project.

Reviewers

@calibration