add function to get euler number

src/reihen.c

@@ -25,3 +25,7 @@ double exponential(double 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