Avoid using the wrong grid by accident

I sometimes have `[mesh:file]` set in the input file, and specify `grid` on
the command line, only to be confused why the wrong grid was picked.

src/mesh/mesh.cxx

@@ -29,8 +29,16 @@ MeshFactory::ReturnType MeshFactory::create(const std::string& type, Options* op
 
   if (options->isSet("file") or Options::root().isSet("grid")) {
     // Specified mesh file
-    const auto grid_name =
-        (*options)["file"].withDefault(Options::root()["grid"].withDefault(""));
+    const auto grid_name1 = Options::root()["grid"].withDefault("");
+    const auto grid_name = (*options)["file"].withDefault(grid_name1);
+    if (options->isSet("file") ond Options::root().isSet("grid")) {
+      if (grid_name1 != grid_name) {
+        throw BoutException(
+            "Mismatch in grid names - specified `{:s}` in grid and `{:s} in "
+            "mesh:file!\nPlease specify only one name or ensure they are the same!",
+            grid_name1, grid_name);
+      }
+    }
     output << "\nGetting grid data from file " << grid_name << "\n";
 
     // Create a grid file, using specified format if given