Add support for relating taxons in protein2GO

[dosumis]

(Moved from JIRA)

Protein to GO should have a new column that allows a choice of host/symbiont relationships between two organisms identified by taxon ID. The relationship will always be in the direction 'expressing/encoding organism -> other organism'. The layout needs to reflect this.

Progress
organism-> organism relations are in RO. Flag 'taxon_extension' is in place. Need to agree the subset of organism-organism relations from RO that curators are happy to use.

@tonysawfordebi (from JIRA ticket) "Discussion is needed at GOC level to determine the requirements for all this multi-organism stuff, and what relations are to be used, and how. Once that is all agreed on, then I can make the necessary changes to P2G.

-> Next step. Revisit in curator meeting.