add function to get euler number

src/reihen.c

@@ -24,4 +24,8 @@ double exponential(double x){
         sum += (1.0/fac(i)*(powerD(i,x)));
     }
     return sum;
+}
+
+double euler(){
+    return exponential(1);
 }

src/reihen.h

@@ -2,5 +2,6 @@
 #define REIHEN_H
 
 double exponential(double x);
+double euler();
 
 #endif // reihen.h