Rename 'Principle' to 'Directive'

The idea of adding an new element 'Directive' to the Motivation Aspect (#99) was discussed and rejected by other forum members. However subsequently I've realized that simply renaming 'Principle' as 'Directive' in the metamodel would solve my problem, which is to do with just having 'Principle' for representing governance concerns.

If that went ahead modelers could simply extend 'Directive' through their own stereotypes to create a governance overlay, which in my case would include 'Principle', 'Policy' and 'Code of Practice'.