Add and link attribute terms: Defines, Identifies, and Indicates. (#1439)

* Add and link attribute terms: Defines, Identifies, and Indicates. Resolves #516.
Co-authored-by: Carolyn MacLeod <Carolyn_MacLeod@ca.ibm.com>
Co-authored-by: James Nurthen <james@nurthen.com>

Author: 3 people