Make input/output accessible by label

[liamhuber]

From our discussion in #102

Programming the nodes with constructs like self.inputs[1] is inconvenient and error prone. It would be nicer to allow constructs with the actual input name, i.e. self.input.alat of self.input['alat']. Being able to provide actual names makes the code much easier to read, avoids errors when introducing new inputs/outputs since it does not require reindexing.

I'm 100% on board with this. Right now the inputs and outputs are just lists, but I should be able to override this with a list-like class that has a modified __getattr__ so it first checks if any of its elements has a label corresponding to the requested key.