input: Write a function named `eval_expr` that takes a string parameter `expr` containing a numerical expression and returns the result of the evaluated expression.
  assert eval_expr("2+3*4")==14
  assert eval_expr("5-2")==3
  assert eval_expr("10/2")==5.0
  assert eval_expr("7")==7
output: def eval_expr(expr):
      return eval(expr)