libpsdxx [WP]: declare LogIndex operator function constant

src/libs/libpsdxx/helper.h

@@ -106,7 +106,7 @@ namespace psd {
          * \param f frequency
          * \return fractional index of frequency \p f
          */
-        double operator()(const double& f);
+        double operator()(const double& f) const;
       private:
         //! factor in front of exponential function in mapping function
         double Ma;

src/libs/libpsdxx/log_index.cc

@@ -57,7 +57,7 @@ namespace psd {
 
   /* ---------------------------------------------------------------------- */
    
-    double LogIndex::operator()(const double& f)
+    double LogIndex::operator()(const double& f) const
     {
      double l = std::log((f-Mb)/Ma)/Mlnc;
      return l;