Task 4: Added logic so that knights gain no XP if their stamina goes below 0

- Knight.java
  - Added new setCanTrain and canTrain method

- Codalot.java
  - Only awarding XP points to knights that can train
  - added logic that sets a knight to no longer train if their stamina goes below 0 and removes their current XP

- KnightTest.java
  - Added unit test to handle the new setCanTrain and canTrain methods

- CodalotTest.java
  - Added unit test to check when a knights stamina goes below 0