Verify target of Real variables with attribute 'derivative'

[filip-stenstrom]

When a variable references what other variable they are a derivative of, no warning/error is produced if the referenced variable cannot have a derivative.

Example of part of XML that doesn't produce any warnings/errors:

<ModelVariables>
    <!-- idx: 1 -->
    <ScalarVariable name="state_var" valueReference="0" variability="discrete" initial="exact">
      <Boolean start="true"/>
    </ScalarVariable>
    <!-- idx: 2 -->
    <ScalarVariable name="der(state_var)" valueReference="1" causality="output" variability="continuous" initial="approx">
      <Real derivative="1" start="1.0"/>
    </ScalarVariable>
</ModelVariables>

<ModelStructure>
  <Derivatives>
    <Unknown index="2"/>
  </Derivatives>
</ModelStructure>

I believe there are two levels of strictness:

1. continuous/discrete Real variables
2. (continuous/discrete) state variables - the target of the derivative attribute has some more limitations (I guess it has to have causality=local/output)