Sort of the input and output variables in the model description

[wbraun]

Currently the Real input and output variables are not sorted together, instead they are arbitrary in the whole Real variable list.

For example the ModelStructure of the DrumBoiler model looks currently:

  <ModelStructure>
    <Outputs>
      <Unknown index="131" dependencies="3 133" dependenciesKind="dependent dependent" />
      <Unknown index="132" dependencies="2" dependenciesKind="dependent" />
      <Unknown index="147" dependencies="3" dependenciesKind="dependent" />
      <Unknown index="162" dependencies="3 133" dependenciesKind="dependent dependent" />
      <Unknown index="163" dependencies="1 2 3 133 161" dependenciesKind="dependent dependent dependent dependent dependent" />
    </Outputs>
    <Derivatives>
      <Unknown index="4" dependencies="2" dependenciesKind="dependent" />
      <Unknown index="5" dependencies="1 2 3 133 161" dependenciesKind="dependent dependent dependent dependent dependent" />
      <Unknown index="6" dependencies="1 2 3 133 161" dependenciesKind="dependent dependent dependent dependent dependent" />
    </Derivatives>
  </ModelStructure>

Here the inputs are 133 161 and the output 131 132 147 162 163.

It would be more efficient for some application if they would sorted simply together.
e.g. inputs:7 8 and outputs:9 10 11 12 13.