[io][brep] add missing const specifier

frackit/io/brepwriter.hh

@@ -76,7 +76,7 @@ public:
     /*!
      * \brief Write the entity network to disk.
      */
-    void write(const std::string& fileName)
+    void write(const std::string& fileName) const
     {
         const std::string brepFile = fileName + ".brep";
         BRepTools::Write(compound_, brepFile.c_str());