Add restrictions to relation tags in relation extraction

[alrichardbollans]

Is your feature request related to a problem? Please describe. Annotating relations and labels in text can become cluttered with lots of incorrect and unnecessary labels.

Describe the solution you'd like One way to cut down on the visual noise is if it was possible to specify that certain relationships can only relate specific types of labels (e.g. in the example here org:founded_by should only be allowed to relate Organisation -> Person. Similarly, other more general restrictions could be useful relating to reflexivity, symmetry etc.