fix declaration of material properties

Author: thelfer

tests/ComputeSpeedOfSoundTest2.cxx

@@ -24,8 +24,8 @@
 int main(const int argc, const char* const* argv) {
   using namespace mgis;
   using namespace mgis::behaviour;
-  const auto yg = 150e9;
-  const auto rho = 7850;
+  const auto yg = real{150e9};
+  const auto rho = real{7850};
   if (argc != 2) {
     std::cerr << "ComputeSpeedOfSoundTest: invalid number of arguments\n";
     std::exit(-1);