Design patterns decisions

Factory pattern (ExpenseGroupFactory)

Create multiple 'Expense group' contract instances to keep track of them and make their management easier
Improve security ensuring each expense group uses identical code and providing an easy lookup mechanism to find a expense group contract.

Withdrawal pattern (ExpenseGroup)

(Pull over push) It places the responsibility for claiming funds, on the recipient of the funds: the recipient has to send a transaction to withdraw and obtain their funds.
This can simplify a smart contract that is sending funds to recipients, because the contract does not have to handle the cases of what to do if sending the funds fail. A smart contract does not know whether sending the funds failed due to an actual error, or whether the recipient is a malicious smart contract that deliberately refuses to accept the funds.

Emergency stop pattern (ExpenseGroup)

Add an option to temporarily stop all contract operations in case of an emergency (due to bugs or security hacks).
For this purpose ExpenseGroup contract is inheriting from Pausable abstract contract (Imported from OpenZeppelin)