Improve link handling (#507)
Improve link handling (#507)

* Make some improvements to link handling to support JBORs

* Make some improvements to link handling to support JBORs

* Add additional documentation; remove support for DXGtable from _guess_link_target_type

dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit