Fix handling of equations without state variables

[JKRT]

model HelloWorldWithoutDer
  Real x( start = 1, fixed = true );
  parameter Real a = 1;
equation
  x = -a * x;
end HelloWorldWithoutDer;

Currently does not work. This is due to the generated code does not match the format of IDA.
Systems like this need to be identified and solved using a simpler scheme.
Currently, there is an issue with symbolic rewriting the variables using Reduce.