follow one definition rule by moving functions into class definition

src/ML-RANN/rann_activation.h

@@ -35,39 +35,19 @@ namespace LAMMPS_NS {
 namespace RANN {
   class Activation {
    public:
-    Activation(class PairRANN *);
+    Activation(class PairRANN *)
-    virtual ~Activation();
+    {
-    virtual double activation_function(double);
+      empty = true;
-    virtual double dactivation_function(double);
+      style = "empty";
-    virtual double ddactivation_function(double);
+    }
+    virtual ~Activation() = default;
+    //default is linear activation (no change).
+    virtual double activation_function(double A) { return A; };
+    virtual double dactivation_function(double) { return 1.0; };
+    virtual double ddactivation_function(double) { return 0.0; };
     bool empty;
     const char *style;
   };
-
-  Activation::Activation(PairRANN *)
-  {
-    empty = true;
-    style = "empty";
-  }
-
-  Activation::~Activation() {}
-
-  //default is linear activation (no change).
-  double Activation::activation_function(double A)
-  {
-    return A;
-  }
-
-  double Activation::dactivation_function(double)
-  {
-    return 1.0;
-  }
-
-  double Activation::ddactivation_function(double)
-  {
-    return 0.0;
-  }
 }    // namespace RANN
 }    // namespace LAMMPS_NS
-
 #endif /* RANN_ACTIVATION_H_ */