input: Write a python function to manage a grocery store inventory, including adding items and getting their prices.
  assert store.get_price('apple') == 1.5
  assert store.get_price('banana') == 0.9
  assert store.get_price('orange') == None
output: class GroceryStore:
      def __init__(self):
          self.inventory = {}
  
      def add_item(self, item, price):
          self.inventory[item] = price
  
      def get_price(self, item):
          return self.inventory.get(item)
  
  # Example usage:
  store = GroceryStore()
  store.add_item('apple', 1.5)
  store.add_item('banana', 0.9)