Improvement of MGA docs + examples, CR3BP example

[MiguelAvillez]

• Overhaul of MGA user guide
• MGA examples: MGA without DSM, MGA with DSM (manual creation of nodes and legs), MGA with hodographic-shaping legs (including manual selection of shaping functions)
• MGA optimization example: MGA without DSM
• MGA optimization example: MGA with hodographic shaping
• CR3BP example: propagation of invariant manifolds of Lagrange point orbit until impact with the surface, in CR3BP-polyhedron model