revert 399b222 (virtual fitness interface temptative)

Incompatible with MOEO's change of interface.

eo/src/EO.h

@@ -72,7 +72,9 @@ template<class F = double> class EO: public eoObject, public eoPersistent
   virtual ~EO() {};
 
   /// Return fitness value.
-  virtual const Fitness& fitness() const {
+  // virtual const Fitness& fitness() const { // This would be impossible with MOEO.
+  // virtual Fitness fitness() const { // Cannot do that either, MOEO changes the interface.
+  Fitness fitness() const {
     if (invalid())
         throw eoInvalidFitnessError("Cannot retrieve unevaluated fitness");
     return repFitness;
@@ -91,7 +93,7 @@ template<class F = double> class EO: public eoObject, public eoPersistent
   /** Set fitness. At the same time, validates it.
    *  @param _fitness New fitness value.
    */
-  virtual void fitness(const Fitness& _fitness)
+  void fitness(const Fitness& _fitness)
   {
     repFitness = _fitness;
     invalidFitness = false;