Set Amira to be the default writer

dune/elasticity/utilities/stresswriter.hh

@@ -111,7 +111,7 @@ void StressWriter<DisplacementField, Grid>::writeStressToFile(const Dune::Parame
 {
   auto stress = getStress(config, displace, grid);
 
-  std::string format = config.get<std::string>("format");
+  std::string format = config.get<std::string>("format", "amira");
   std::transform(std::begin(format), std::end(format), std::begin(format), ::tolower);
 
   if (format == "amira") {