add tangens function

src/reihen.c

@@ -79,4 +79,15 @@ double cosD(double degrees){
         else sum -= currvalue;
     }
     return sum;
+}
+
+double tanD(double degrees){
+    if (degrees>=360.0){
+            degrees = squashDegreesTo360(degrees);
+    }
+    if(degrees == 90.0 || degrees == 270.0){
+        return -1.0;
+    }
+    return sinD(degrees)/cosD(degrees);
+
 }