support property:GRAPHICS

[ehenneken]

This PR supports the ability to run the query property:GRAPHICS to retrieve all records with an entry in the graphics database or to check whether a record has graphics associated with it. The main reason for this functionality is for Nectar to check whether the graphics tab should be enabled in an abstract view.

• The updates in test_reader.py check whether data is properly read
• The updates in test_process.py check whether the value GRAPHICS ends up in the property field
• A stub data directory config/links/graphics has been created
• The field graphics has been added to the list of fields that do not go into the protobuf