refactoring: if there is only one number just return that one as the average

src/average_median/average_median.c

@@ -7,6 +7,9 @@ double average(const double *data, int n) {
     if (n==0) {
         return 0;
     }
+    else if (n==1){
+        return data[0];
+    }
     else {
     double sum = 0.0;
     for (int i = 0; i < n; ++i) {