generated c++ code needs should be compatible with c++17 and above

[fbergmann]

As discussed in this issue: sbmlteam/libsbml#36, the current libSBML only compiles with c++ standard up to 14. This is because of the use of unary_function and ptr_function. The code compiles fine without these, so all that needs doing is not to inherit from unary functions.

Deviser generates them for finding elements by id or reference:

 /**
  * Used by ListOf::get() to lookup an SBase based by its id.
  */
 struct IdEqS : public unary_function<SBase*, bool>
 {
   const string& mId;
 
   IdEqS (const string& id) : mId(id) { }
   bool operator() (SBase* sb) 
        { return static_cast <Species *> (sb)->getId() == mId; }
 };

this struct will work just as well, by not using the unary_function.