Fix namespace conflict on math nodes

[avandecreme]

Description

This is an attempt to fix #316. I am not super happy with that fix because it effectively makes the sbml namespace reserved but I don't have a better idea.

Motivation and Context

Fix #316

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically

[fbergmann]

thank you for this. We should indeed not add two sbml namespaces.