Added C interface

example.c

@@ -0,0 +1,26 @@
+#include <stdlib.h>
+
+void etics_external_init(const int n_total);
+void etics_external_grav(const double* const A, const int n_act, double x_act[][3], double pot_act[], double a_act[][3]);
+
+int main()
+{
+    const int n_total = 1024;
+    etics_external_init(n_total);
+
+    srand(19640916);
+
+    double x_act[n_total][3], pot_act[n_total], a_act[n_total][3];
+    for (int i = 0; i < n_total; i++) {
+        x_act[i][0] = rand();
+        x_act[i][1] = rand();
+        x_act[i][2] = rand();
+    }
+
+    double A[2048];
+    for (int i = 0; i < 2048; i++) {
+        A[i] = rand();
+    }
+
+    etics_external_grav(A, n_total, x_act, pot_act, a_act);
+}