Refactor Exposure classes and adjust tests

[SZeltaat]

Description

Refactored and replaced exposure classes. Main change is the definition of ABCEnumMeta class to be able to make Exposure an abstract class. This was required if we want to define the concept of exposure class independant of the exact definition in different Eurocode versions.
I had to do this in order to be able to do the same for the structural class and the cover calculation.
All the other changes are mostly replacing code.

Please also read #305
Fixes #305

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

[github-actions]

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[egarciamendez]

Beautiful !

[SZeltaat]

Thank you for your review!
I tried changing 'object' to 'T', but mypy complains that the signature is different from its superclass. I have tried a couple of other possible solutions, but with no luck. So I'm keeping 'object' in. If you can think of a solution, please let me know or feel free to apply it yourself!
I'll wait for your response before merging! 😉

[egarciamendez]

Sometimes is a 9 just fine 👌