attributes for links

Hi Ross,

Right now links do not carry links but this could happen in the future.  I have seen customers with DOORS 9 use a additional module between the source and target to hold additional information about traceability, often the justification of why the link exists and perhaps evidence of the analysis that was done.  It's somewhat arduous though as you end up with two links and an artifact to represent what you would normally just have a single link for.  Where I have seen this working customers have had two different link schemas, normal links going directly and justification links going indirectly through an additional module.