Bad treatment of algebraic loops in synchronous models

[rfranke]

​PR1391 along with ​commit d32ecb make ControlledMixingUnit work by applying index reduction and appropriately implementing ExplicitEuler, respectively.

Some more issues show up when replacing ExplicitEuler with ImplicitEuler, introducing algebraic loops. See the following example:

model ControlledMixingUnit2
  extends Modelica_Synchronous.Examples.Systems.ControlledMixingUnit(
    periodicClock1(solverMethod="ImplicitEuler"));
end ControlledMixingUnit2;

ImplicitEuler leads to the introduction of algebraic loops. No Jacobians and no ModelStructure are generated though!

Moreover, there is a linear torn system of dimension 1 -- shouldn't this be solved analytically?

Last but not least, the C runtime does not generate code for algebraic loops in clocked partitions.