Update override handling

[syntron]

This is an update of PR #400 based on my comments:

• move import re to the right point
• call version parsing only once (directly in __init__())
• merge override code in one function and call it as needed - def _process_override_data()

This needed some cleanup for _linearization_options and _optimization_options. Up to this PR, these were defined as dict[str, str | float] with the float option only used on initialisation. As soon as OMC was run, the data would be defined as str. Always forcing str removes this ambiguity and simplifies the definitions. This change is within the same PR as it is needed to get an OK from the linters ...