Sources for tools/nodes should be dicts not lists

[tdiethe]

Would make it easier to check for errors, and remove special cases such as alignment_node and splitting_stream. However would require all tools to be rewritten.

[perellonieto]

I will help after I create the first tutorial that uses a source.

[tdiethe]

I think we came to the decision to close this one: it would be a huge change and potentially not backwards compatible with existing data.