add logarithmic function for all bases

src/logarithmus.c

@@ -4,6 +4,13 @@
 #include "logarithmus.h"
 #include "exponent.h"
 
+double logX(double b, double a){
+    if(b == 1.0 || b < 0.0){
+        return -1.0;
+    }
+    return(logN(a, 0.000001)/logN(b, 0.000001));
+}
+
 double logN(double x, double eps){
     if(x <= 0){
         return -1.0;

src/logarithmus.h

@@ -2,5 +2,6 @@
 #define LOGARITHMUS_H
 
 double logN(double x, double eps);
+double logX(double b, double a);
 
 #endif // logarithmus.h