Add Rhea column for edges

[DeniseSl22]

Since some time, we have PWs with identifiers on the edges, see for example WP4518 . If these IDs could be added as an additional column(s) (XrefID and Xref column), we could also link fluxomics data to the PWs with more ease (at a later stage, working on mapping kinetic data to our PW models now in RDF). For all the Reactome PWs, we also have IDs on the interactions (from Reactome). They are working on changing this to Rhea (since EBI wants their resources to work together more). So maybe at some point we will have Rhea IDs for the PWs too. Besides this, in BridgeDb we have mapping files for interactions (now also part of our API). So at a later stage, we could also add a mapping option for this column. Hope I explained my question clear enough, if not please let me know @AdamStuart @AlexanderPico

[DeniseSl22]

@egonw @mkutmon as just discussed in our BiGCaT group meeting.