Io nodes refactor

Since I was investigating #266, I took the opportunity to refactor the conversion from graphInput/graphOutput in ewoks input_nodes and output_nodes.

Mostly, I tried to improve the readability by changing names and splitting functions that were trying to deal with input and output as the same time. The code should be functionally the same but I think it should be a little faster since a few loops were removed.

The diff is quite heavy since there is a lot of renaming and moving things around so you can either review commit by commit or gloss over the review.