Added naturalLogarithmFunction (ln) for natural logarithmic calculations

src/main/c/main_taschenrechner.c

@@ -45,6 +45,11 @@ double logarithmFunction(double x) {
     return log10(x);
 }
 
+double naturalLogarithmFunction(double x) {
+    // Natural logarithm (ln)
+    return log(x);
+}
+ 
 
 
 // scientificmode

src/main/c/taschenrechner.h

@@ -9,14 +9,19 @@ int multiply(int a, int b);
 
 int divide(int a, int b);
 
+// Trigonometric functions
 double sineFunction(double angle);
 
 double cosineFunction(double angle);
 
 double tangentFunction(double angle);
 
+//..
+// Logarithmic functions
 double logarithmFunction(double x);
 
+double naturalLogarithmFunction(double x);
+
 int mode(int userChoice);
 
 int displayMenu();