Examples 10.73 and 10.74 from Betts' book

[Peter230655]

These two examples solve the same problem, but formulated differently.
This may be a candidate for examples-gallery for these reasons:

• 10.73 has a boundary condition at the final time: function(state variables, parameters) = 0. As opty currently does not allow such instance constraints, I introduced an additional state variable, hilfs, and an additional equation of motion as hilfs - function(state variables, parameters). Then I set an instance_constraint as hilfs($t_f$) = 0.
  I believe there is no such example in examples-gallery.
• while 10.74 converged very rapidly, 10.73 only converged with very close initial guesses. This shows, that the formulation of a problem has a (big) influence on the ease of solving it.