Add instance check of export_vars

[HoeppJ]

What is the problem?

Method process_export_vars in project.py does not check if given items of dict export_vars are an instance of a list.

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Why do we want to solve it?

Returned string export_vars_str of method process_export_vars would lead to incorrect output variables in the dymola result file

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How do we want to solve it?

Add code in the method process_export_vars to check, if all items of given dict export_vars are an instance of a list.
Raise error, when an item is not an instance of a list.

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