Allow connection of output of step to (specific) input var of another step

[raar1]

Currently we simply set precedence relations between steps (Step A precedes Step B).

Instead we need to be able to specify that the output of step A is bound to a specific input variable of Step B.

Blocked by #124

[raar1]

I have implemented this (loosely) in PR #146 with triples that 'bind' the output of one step to a specific input of another. However, I generate this from the noodles workflow inside the from_noodles_promise class method, and there is not yet a standard method that allows the connections to be specified. Furthermore, the predicate I am using is made up - we will hopefully learn what the correct way to specify all of this is in the upcoming hackathon (26th Jan).