Add a human-readable representation of the objects

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 It is convenient, especially in a Python console, to just type a variable and get a human-readable representation of this variable.

 The idea would be to have a toString() method in the C++ classes, and a __repr__ function in the Python interface for every single class.
+
+**Note**: Relegated to very low priority. Reasons:
+
+* It feels strange to stuff these very small objects with toString() methods.
+* Often, it is rather cumbersome to give a useful string representation, or it is not entirely clear how to make the most sense out of it (sum of operators, for examples, can be large).
+* Neither the data model nor the scripts are so complex that not knowing what you have in your hand should usually be an issue.