#347: Fix terminal background in configuration file

[tlamonthezie]

Fixes #347

• This pull request remove the terminal_background option from the configuration and from the existing configuration files. Note: we might need to check again for #154 . If the problem of #154 occurs again it should be resolved by client user by changing the terminal settings. That is because a logged message is written to the console after a "clear color" so it should use the default user terminal color. If color is bad then change the client user terminal text color
• terminal_background option has also been removed from rst docs and from the schema defined in the ConfigurationValidator class
• Additionnaly it improves the configuration_upgrader.py script to keep comments at the top of the file if defined
• It also adds the display of an error message and exits lbaf if running with Python 3.9 in case save_meshes is True in configuration file from the Visualizer. This is to explicitly say that this error is known.
• LBAF_app refactoring for more readability : load arguments inside a dedicated method
• Fix some configuration parameters not classified in sections in configuration files when using configuration upgrader

[tlamonthezie]

Changes requested

Thank you @ppebay should be now ok