Added all the global variables required in CarPark.java
also add simple return statements to non-void method so no error msg
would pop up

@Chun Chan
Started a little parts of tests for CarParkTests
Add more tests for carParkTest

contain bug that keep adding vehicles with same vehID, still thinking of
solution for this.
Previous bug fixed for CarParkTests
Modified enterParkedState() and its test

according to revised version of javadoc, parkingTime <=0 require a
throws of VehicleException
close to complete version for CarParkTests

still figuring out about two methods' tests
Complete version for CarParkTests

finishing up the remaining two methods' test
Tests completed and minor modified of Vehicle