More multisystem messages changes

Description

Several changes to handle output in multisystem mode:

• Change several routines to take an I/O unit and a namespace as optional arguments.
• Systematically pass the namespace instead of the stdout unit number to messages_print_stress.

News snippet

Several changes to handle output in multisystem mode.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.