Feature: Possibility to handle the user config from the callbacks script

Description of change

• Split read_user_config into a load_user_config to allow to read the user config without loading it into Stax.
• write_user_config takes the current user config as base dict and takes an argument to add additional_data if wanted. Allows the callbacks script to insert its own data in the config in order to use them.

How to test it

• Use the kistu demo and change the settings of the load episode, validate
• exit
• start again, it's been kept.

Checklist

Please check if your MR fulfills the following requirements:

• Unit tests have been added, if needed
• Documentation have been updated, if needed
• Build was run locally and any changes were pushed
• Lint has passed locally without error
• Package.py have been updated, if needed
• CHANGELOG.md have been updated, if needed
• Version numer has been tagged, if needed

FYI: @awesome_dev_too