GeoGenericFunctions: Fix clang warnings.

clang warnings: missing/inconsistent override keywords.

GeoGenericFunctions/GeoGenericFunctions/ACos.h

@@ -31,14 +31,14 @@ namespace GeoGenfun {
     ACos(const ACos &right);
   
     // Retreive function value
-    virtual double operator ()(double argument) const;
-    virtual double operator ()(const Argument & a) const {return operator() (a[0]);}
+    virtual double operator ()(double argument) const override;
+    virtual double operator ()(const Argument & a) const override {return operator() (a[0]);}
 
     // Derivative.  
-    Derivative partial (unsigned int) const;
+    virtual Derivative partial (unsigned int) const override;
 
     // Does this function have an analytic derivative?
-    virtual bool hasAnalyticDerivative() const {return true;}
+    virtual bool hasAnalyticDerivative() const override {return true;}
   
   private: