456 Add formula 6.63 implementation and tests from NEN-EN 1992-1-1+C2:2011

[GerjanDorgelo]

Description

adds eq 6.63

Fixes #456

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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