Multisystem support for messages: enable namespaces

Description

Add multisystem support for messages by adding an optional namespace argument for messages_fatal, messages_warning, messages_not_implemented, and messages_print_stress. If this is present and non-empty, the namespace is also printed to stdout. This is done to be able to trace back from which system a message originates.

These functions are changed in the middle-level part of the code: in the folders system, states, hamiltonian, species, and ions.

News snippet

Add multisystem support for messages

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.

Closes #190 (closed).