Should Pipeline.visualize default to show the full graph if no keys are given?

[SimonHeybrock]

See title. I think this would be quite useful.

[jl-wynen]

Comment from #181:

We have a number of pipelines across multiple projects now. And we compose new pipelines from them. It is currently very difficult to inspect a pipeline in its entirety to, e.g., find out what it can produce, or how it does a certain operation. It would be a great help if we could just visualize a pipeline without having to already know what it can produce or what products depend on the subgraph you are currently interested in.

[jokasimr]

Fixed in #188