add beyond convexity to theory chapter

[Jonas Hübotter]

• continuity
  • always continuous?
  • continuity at boundary?
    • https://math.stackexchange.com/a/584739/639214
• differentiability (also called smoothness)
• Lipschitz
• smoothness
• strong convexity
• locally polyhedral

• check if model fulfills properties (find variants that fulfill properties)
• constant and linear functions are not strongly convex (or locally polyhedral)