generalize ff_controller and extended_controller to accept any system

[baggepinnen]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.48%. Comparing base (7bab236) to head (a2b5d53).

Files with missing lines	Patch %	Lines
src/lqg.jl	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   90.49%   90.48%   -0.02%     
==========================================
  Files          19       19              
  Lines        2968     2975       +7     
==========================================
+ Hits         2686     2692       +6     
- Misses        282      283       +1     

Flag	Coverage Δ
unittests	90.48% <80.00%> (-0.02%)	⬇️

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