Standardise approach for string to enum conversion

src/operations.h

@@ -45,6 +45,11 @@ namespace sharp {
    */
   VImage Gamma(VImage image, double const exponent);
 
+  /*
+   * Flatten image to remove alpha channel
+   */
+  VImage Flatten(VImage image, std::vector<double> flattenBackground);
+
   /*
    * Produce the "negative" of the image.
    */