ORM redesign: implement link type definitions

[sphuber]

In the current design, we have two "planes" of provenance, the "creation" and the "logical" plane, with CalculationNodes inhabiting the first and WorkflowNodes inhabiting the second. The links in these planes also will have distinct names although there being overlap in behavior. In the future, we most likely will introduce a third plane where users can create arbitrary connections through a custom type link. Since we don't want to support an infinite amount of link types, we will probably use a single custom link type, that users can then further sub classify with a sub type. This will require the link table to get another column sub_type in addition to type. This actually plays very nicely with the existing link types.
Proposal:

type	sub_type	Connections	Old name
INPUT	calc	Data -> Calculation	INPUT
INPUT	work	Data -> Workflow	PASS
OUTPUT	create	Calculation -> Data	CREATE
OUTPUT	return	Workflow -> Data	RETURN
CALL	calc	Workflow -> Calculation	RUN
CALL	work	Workflow -> Workflow	CALL
CUSTOM	?	?	-