input: Write a python function to calculate the total price including tax given the price, quantity, and tax rate.
  assert calculate_total_price(10, 2, 0.08) == 21.6
  assert calculate_total_price(15, 3, 0.1) == 49.5
  assert calculate_total_price(5, 5, 0.05) == 26.25
output: def calculate_total_price(price, quantity, tax_rate):
      subtotal = price * quantity
      total_price = subtotal * (1 + tax_rate)
      return total_price