Implement basic Coin. First test passes
Add test for negative coin denominations
Add exception. second test passes.
Add first Coin changer test
Implement basic CoinChanger constructor. test passes
Add basic test for get_change.
test passes
Add test for denomination array with 2 elements
Reimplement get_change, test passes
Add test for different amount
Test passes
Added more complex test
test passes
Add 1 more test
Move repeated test_coin_changer creation into setup
Removed unused coin and coin_spec