Add theory reference in docs

[mortenengen]

This is a collection of topics that we should cover in the theory reference in the docs.

• Sign convention for cross sections, Improve documentation for sign criteria #137, calculate_bending_strength results - coordinate axes #163, Error in computation of bearing capacity for axial loading #140, My-Mz calculation discrepancies for N=0 in cross section #141, N-M calculation discrepancies in cross section #142, Discrepancies in Moment-Cuvature results #144.
• Describe that stress resultants are integrated in y- and z- direction, and if the section is created with the origin displaced from its center of gravity, this will introduce an eccentricity moment.
• Constitutive laws:
  • Describe the implemented constitutive laws.
  • Describe getting stresses and stiffnesses from constitutive laws.
  • Describe getting and setting the constitutive laws on materials.
• Describe reinforcement properties, and how these can be used for selecting groups of reinforcement (Improving smooth access to certain reinforcement areas in concrete sections #146):
  • name
  • group_label
• Describe fiber integrator.
• Describe Marin integrator.
• Describe steps in calculating:
  • Limit axial load
  • Integrate strain profile
  • Bending strength. Including how to calculate the strengths about the two main axes (Inconsistent Bending Strength Calculation for Rotated Sections #150).
  • Moment-curvature
  • Interaction domains. Describe that it is the interaction domain for the negative moment that is returned by default (N-M calculation discrepancies in cross section #142)
• Desbribe the common interface defined by material classes ([enhancement] Create Abstract Class for Codes  #198)