Relation modules

Specifications of a bundle of relations and structures modules. They naturally are tightly connected to SQL, so can be viewed as a kind of SQL query builder.

Goals

explicit relations module
- provide a unified db structure for explicitly related things
- provide a set of functions to get and set relations
- provide nodeapi extension
tree relations module
- provide a db structure for nested set indexes
- provide setter and getters for those
- provide nodeapi extension
related by terms module
- implement the relations api
- provide a db interface
general relations module
- provide a relations api
- provide a set of relation operations - addition, difference
- provide a relations node