Extend material definition with additional fields

[stfnp]

As a preparation for #90, add the following fields to the materials:

• Source
• Description
• Tensile strength
• Compressive strength

Use tensile and compressive strength for #64

[stfnp]

Maybe use this opportunity to also add the shear modulus or the Poisson ratio. This might eventually be needed for torsional stability, see #106.