add atan function

src/reihen.c

@@ -81,3 +81,17 @@ double tanD(double degrees){
     return sinD(degrees)/cosD(degrees);
 
 }
+
+double atanD(double degrees){
+    if (degrees>=360.0){
+            degrees = squashDegreesTo360(degrees);
+    }
+
+    double sum = 0.0;
+    double radian = radians(degrees);
+    for (int i = 0; i <= 9; i++){
+        sum += powerD((double) i, -1.0) * (powerD(2.0*i+1, radian) / 2.0*i+1);
+    }
+
+    return sum;
+}