Operations added

src/main/java/rocks/zipcode/calcskin/CalcEngine.java

     }
 
     public double add(double v, double v1) {
-        return Double.NaN;
+        return v + v1;
     }
 
     public double subtract(double v, double v1) {
-        return Double.NaN;
+        return v - v1;
     }
 
     public double multiply(double v, double v1) {
-        return Double.NaN;
+        return v*v1;
     }
 
     public double divide(double v, double v1) {
-        return Double.NaN;
+        return v/v1;
     }
 }