add tangens function

src/reihen.c

@@ -80,3 +80,14 @@ double cosD(double degrees){
     }
     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);
+
+}