Added abstract ContainerHolder, implemented it in Transporter and Agv
Added documentation to Transporter
Added Storage class
Replaced setter statement with setter method
Added unit testing for Storage
Implemented ContainerHolder in Storage
Merge branch 'containerholder'

Conflicts:
	src/main/java/org/nhl/containing_backend/vehicles/Agv.java
	src/test/java/org/nhl/containing_backend/vehicles/TestAgv.java