refactoring: if angle is 0 return 0 for tangens

src/trig_functions/trig_functions.c

@@ -12,5 +12,6 @@ double calculate_cos(double angle) {
 }
 
 double calculate_tan(double angle) {
-    return tan(angle);
+    if (angle==0) return 0;
+    else return tan(angle);
 }