WIP allow mutliple tokens to be selected via the linker widget
Allow for multiple linked_agents to be added via the linker (cloning all other fields for each linked_agent association record)
Only allow mutlitple subjets/agents to be added via the linker for new associations
Update linker label to reflect the multiplicity setting
Merge pull request #627 from hudmol/linker-with-multiple-items

[Delivers #51842627] Linker with multiple items