Implement Structure related queries to database object

[skiadas]

The db module should offer structure-based queries:

• [ ] Return a structure based on its id
• [ ] Return all structures of a specific type (e.g. workspaces or graphs) for a specific user
• [ ] Create a new structure
• [ ] Delete an existing structure
• [ ] Update an existing structure
• [ ] Return all structures that "belong" to a particular structure (e.g. objects linked in a workspace)