Namespace improvements

Description

This MR includes a few different improvements to the handling of namespaces:

• namespaces can now have parents
• the code stops if a namespace is too large
• there is now a global namespace
• the messages_error_input now requires a namespace so that the user knows exactly which input variable is incorrect.

News snippet

Several improvements to the handling of namespaces.

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.