OCCI Core

- improved handling of parent attribute especially in OCCI Category function get_class
- some small fixes and improvements to kind and mixin classes regarding their instance attributes
- fixed instantiation of Mixins in the Mixins Collection class
- the fixes in the get_class function in the Category class and in the convert function in the Mixins class now lead to much cleaner handling of Mixins
- this pull request is now feature complete, the rspec tests are passing locally, but further testing is needed to see if there are side effects resulting from the changes