Implemented slice unit as a resource

This pull request contains the following implementation aspects:
1. Created the unit as a resource
2. Moved management from `ISliceAdministration` to `IUnitManagement` and `IUnitAdministration`
3. Added slice and unit wrappers
4. Adapted logic in `SliceAdministration`
5. Removed slice unit definition from `Specification`
6. Adapted tests