Restrict usage of isSimilarTo

Need to show where ceterms:isSimilarTo is appropriate.  Currently it has no declared domain and so is available for use on any entity type but appears on none.  

Change this so that its domain includes:

- Credential classes,
- Learning opportunity classes & assessments
- Industry, Occupation, Job, Task, Role
 
Maybe 
- Actions
- Support service
- Rubrics