[ENH] Implement cross-section classification of steel sections following eurocode 3

**Is your feature request related to a problem? Please describe.**
The class of the cross-section is an important property that influences the procedures for the verification of the resistance of the steel cross-section and the member. This feature has not yet been implemented in this module, and is restricting the further development of the strength verification procedures.

**Describe the solution you'd like**
A function or class method that takes four arguments (a steel section (e.g. `SteelSection`), a steel material (e.g. `BasicStructuralSteel`), and the design axial force and bending moment acting on the cross-section) and return an integer from 1-4 representing the four possible cross-section classes.