DRAFT: for ENH/3-dof-simulation (See RocketPy-Team#655)

ENH: adds 3 DOF simulation capability to rocketpy.Flight.

*ENH: added "u_dot_3dof" for "solid_propulsion"
mode

*ENH: adding "u_dot_generalized_3dof" for "standard"
mode (still incomplete)

*ENH: new parameter "simulation_mode" for swtiching
between 3 dof and 6 dof

*ENH: updated conditions for "__init_equations_of_motion"

*ENH: 2 new example files have been created to test 3 dof model
"test_bella_lui_flight_sim" and "test_camoes_flight_sim"

Author: aZira371