Reorganize Java files to correct paths

...anagment/src/main/java/uk/ac/rhul/cs2800/model/Grade.java

...nagment/src/main/java/uk/ac/rhul/cs2800/model/Module.java

...t/src/main/java/uk/ac/rhul/cs2800/model/Registration.java

...agment/src/main/java/uk/ac/rhul/cs2800/model/Student.java

...ment/src/test/java/uk/ac/rhul/cs2800/model/GradeTest.java

...ent/src/test/java/uk/ac/rhul/cs2800/model/ModuleTest.java

...c/test/java/uk/ac/rhul/cs2800/model/RegistrationTest.java

...nt/src/test/java/uk/ac/rhul/cs2800/model/StudentTest.java