Global section is not written to the roughness-channels.ini file

[priscavdsluis]

Describe the bug
The Global section is not written anymore to the roughness-channels.ini file.
The global field is a special case because global is a built-in keyword and thus is escaped by appending it with an underscore: FrictionModel.global_

A missing global section leads to an invalid model.

To Reproduce
Steps to reproduce the behavior:

1. Create a new FrictionModel
2. Save the model to a location

Expected behavior
There is a [Global] section in the file

Additional context
It was discovered during the preparation of the DSD 2024.
It used to work during the DSD 2023.