Added a multi-layer cloud system example
Added Alloy themes to show instances for OCCI platfrom and application
models.
- Updated how URI is defined, used a macro instead of a sig.
- Removed the identity relation and replace it by 
  predicates sameIdentity and sameCategories.
These 2 updates strongly reduce Alloy analysis execution time.
Corrected an inconsistency: duplicate term for an action.
Added the multi-layer cloud system example
Merge remote-tracking branch 'origin/master'

Conflicts:
	alloy.md
merge
Alloy theme for the multi-layer cloud system example.