Fix dynamics calculation of GenericSystem component (backport #2705)

[mergify]

There were several issues, introduced with #2571:

• as reported in Generic system assumes acceleration interface, though none is present #2694, the component tried to set states even if they didn't exist
• while writing test cases for the above, I realized that the combination of position-only state with velocity-only command was not working, too. The problem here was that the internal variable of the velocity state was never set and thus, no integration happened. The behavior change came from the fact that the initial value of states is NaN if not explicitly set, and in this case there is no velocity state which could be parameterized.
• The comment "backward integration" was wrong, backward Euler integration is $x(k+1) = x(k)+T_s f(k+1)$, but forward euler integration $x(k+1) = x(k)+T_s f(k)$ was implemented. Doing real implicit Euler fixes the above, but changes obviously the behavior a bit, see the changes in simple_dynamics_pos_vel_acc_control_modes_interfaces. But IMHO this is fine, unless anyone has implemented a similar test pattern with this component.

Fixes #2694

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This is an automatic backport of pull request #2705 done by Mergify.