Add a phase to DAELow to check if a variable (cref) used into an equation/algorithm actually exists in the declared variables. Also we could check if all declared variables appear in any equation/algorithm.

[Adrian Pop]

It would be easy to detect bad equations if we have a phase (or add it to lower)
in which we check that a variable (cref) used in an equation or algorithm is actually
present in the declared variables.
This will make it easier to detect which components wrongly do not appear in the DAE even if
they should or which equations/algorithms should not appear in the DAE but they wrongly do.

[Jens Frenkel]

resolved in revision 6568