Catch polymorphic exception by value

gcc 8 warns about this.

src/finite-strain-elasticity.cc

@@ -359,6 +359,6 @@ int main (int argc, char *argv[]) try
     vtkWriter.write(resultPath + "finite-strain_homotopy_" + std::to_string(i+1));
   }
 
-} catch (Exception e) {
-    std::cout << e << std::endl;
+} catch (Exception& e) {
+    std::cout << e.what() << std::endl;
 }