MC2020: Material properties of concrete

[mortenengen]

Implement code related to material properties of concrete:

• Compressive strength (Ch. 14.6.1.1)
• tensile stress (Ch. 14.6.1.2)
• modulus of elasticity (Ch. 14.6.1.4)
• stress-strain relations (Ch. 14.6.1.5)
• time effects (Ch. 14.6.1.6)
• temperature effects (Ch. 14.6.1.7)