Fix dynamics calculation of GenericSystem component

[christophfroehlich]

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

[codecov]

Codecov Report

❌ Patch coverage is 97.91667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.40%. Comparing base (6d27e9d) to head (a506539).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...e_interface/src/mock_components/generic_system.cpp	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2705      +/-   ##
==========================================
+ Coverage   89.33%   89.40%   +0.07%     
==========================================
  Files         151      151              
  Lines       17147    17225      +78     
  Branches     1432     1431       -1     
==========================================
+ Hits        15318    15400      +82     
+ Misses       1247     1246       -1     
+ Partials      582      579       -3     

Flag	Coverage Δ
unittests	89.40% <97.91%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rface/test/mock_components/test_generic_system.cpp	99.76% <100.00%> (+0.05%)	⬆️
...e_interface/src/mock_components/generic_system.cpp	86.33% <87.50%> (+1.54%)	⬆️

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