Default __call__ of Nodes
A new change I will introduce is that the Nodes call function is not a dask.delayed function by default. This has already been partially implemented in the develop branch, but still needs to be finished.
The intuition is that the nodes should be usable without having to work with a dask graph. There will be an option (global and per node) which enables dask.delayed wrapping of the call function. @tkuonen will need to consider this when implementing the visualization.