Add to_string to AffExpr and QuadExpr

[mpowelson]

This would be useful for debugging. There is currently code for AffExpr in modelling_utils

[arocchi]

There is also operator<<(std::ostream& o, const AffExpr& e) and operator<<(std::ostream& o, const QuadExpr& e) - these could be refactored and split between an operator<< and a str or to_string method - the function you mentioned looks like code duplication with respect to operator<<(std::ostream& o, const AffExpr& e)?