Implement nominal concrete cover check

[SZeltaat]

Description

• Check class implemented as a subclass to Formula. Is that acceptable? Or do we want to avoid this?
• The check depends strongly on the release year and the national annex of the Eurocode. Current implementation is semi-flexibel (all specific values are stored in a separate file, but are referenced directly in the check, so no choice for other EC version implemented ). We might want to come up with a protocol for that!
• Eurocode allows for marginal reduction of cover (Δc_dev to be precise). That is not implemented. I've never seen it being used and the conditions are too specific.

Progress

• Check class/function for nominal concrete cover
• implementation of design allowance for deviation
• LateX report
• tests
• example for documentation

Fixes #343

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

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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