input: Write a python function to create a BankAccount class with deposit and withdraw methods.
  assert account.deposit(1000) == 1000
  assert account.withdraw(500) == 500
  assert account.deposit(200) == 700
  assert account.withdraw(700) == 0
output: class BankAccount:
      def __init__(self):
          self.balance = 0
  
      def deposit(self, amount):
          self.balance += amount
          return self.balance
  
      def withdraw(self, amount):
          if amount <= self.balance:
              self.balance -= amount
              return self.balance
          else:
              raise ValueError("Insufficient funds")