Workflow: Build IO in the property if it doesn't exist already

[liamhuber]

This brings it into better alignment with Workflow, where Input and Output are properties (returning the correct type over in #729).

I expect this to make it easier to make an abstract parent for both Workflow and Node, because now such a parent can just expect self.inputs and self.outputs to give back a class of the right type at all times.

[github-actions]

👈 Launch a binder notebook on branch pyiron/pyiron_contrib/delay_io_creation

[github-actions]

Pull Request Test Coverage Report for Build 5382199053

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 11.946%

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Totals
Change from base Build 5382174367:	0.06%
Covered Lines:	1680
Relevant Lines:	14063

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💛 - Coveralls