Added Dehnen potential and moved external potentials to a new compilation unit

external.h

@@ -0,0 +1,59 @@
+#pragma once
+#include "double3.h"
+
+class External_gravity {
+public:
+    void apply(const int n_act, const double3 x[], const double3 v[], double pot[], double3 a[], double3 adot[])
+    {
+        for (int i=0; i<n_act; i++) {
+            this->set_coordinates(x[i], v[i]);
+            this->calc_gravity();
+            pot[i]  += potential;
+            a[i]    += acceleration;
+            adot[i] += jerk;
+        }
+    }
+    virtual void calc_gravity() = 0;
+    bool is_active;
+protected:
+    double potential;
+    double3 acceleration, jerk;
+    double3 x, v;
+    void set_coordinates(double3 x, double3 v)
+    {
+        this->x = x;
+        this->v = v;
+    }
+};
+
+class Plummer : public External_gravity {
+public:
+    Plummer(double m, double b) : m(m), b(b) {is_active=(m>0);}
+    void calc_gravity();
+private:
+    double m, b;
+};
+
+class Miyamoto_Nagai : public External_gravity {
+public:
+    Miyamoto_Nagai(double m, double a, double b) : m(m), a(a), b(b) {is_active=(m>0);}
+    void calc_gravity();
+private:
+    double m, a, b;
+};
+
+class Logarithmic_halo : public External_gravity {
+public:
+    Logarithmic_halo(double v_halo, double r_halo) : v2_halo(v_halo*v_halo), r2_halo(r_halo*r_halo) {is_active=(r_halo>0);}
+    void calc_gravity();
+private:
+    double v2_halo, r2_halo;
+};
+
+class Dehnen : public External_gravity {
+public:
+    Dehnen(double m, double r, double gamma) : m(m), r(r), gamma(gamma) {is_active=(m>0);}
+    void calc_gravity();
+private:
+    double m, r, gamma;
+};