DOMAIN OBJECTS - IDSet: Change method

Changed how IDSet does differences from polynomial time to linear time.
TESTS - Tests for IDSet

Forgot to commit tests with last commit.
REFACTORING - IDs : Standarized how IDs are done

Standarded how IDs are handled and asserted. Database and System had
different assumptions. Everything uses the Database's assumptions now.
REFACTORING - IDs: Missing File

Added file that should had been committed with last commit.