Defining interfaces to collection classes

Interfaces are defined for the AssetFileCollection, AssetCollection and
LocatorCollection classes.
Added comments to the methods in the interfaces
Revert "Added comments to the methods in the interfaces"

This reverts commit 2fde602c44b376c1dfd20d9c2a7e3f80de92c8a9.
Revert "Revert "Added comments to the methods in the interfaces""

This reverts commit b8d6a6553341a8a14ab811fcd5659dc4ab81a807.