By Arathy Somasekhar, Alexandra Ulmer and Daniel Trotta HOUSTON (Reuters) – America’s biggest gun lobby, the National Rifle Association, pressed ahead with preparations for its annual gathering starting on Friday in Houston, just 280 miles (450 km) from the scene of the country’s worst school shooting in a decade. An 18-year-old gunman armed with a semi-automatic rifle that he was legally able to purchase stormed an elementary school in Uvalde, Texas, on Tuesday, killing 19 children and two teachers in a hail of bullets. The NRA gathering takes place this year in the shadow of not one but two …