[patch] Give every node `SingleValue` power

[liamhuber]

Pulls SingleValue functionality out and let all nodes inherit from it as a new mixin. This allows us to use all the node injections on macros and workflows (as long as they only have one output channel), and means you don't need any special class to make a funciton "single value" -- as long as it's only got a single return value it will act this way be default. (That means we can even get rid of the SingleValue class, but that is a separate PR).

This change ran into an edge case where function nodes use self to store data. I solved this by adding new dict field to store it in -- since the field is pre-existing, we bypasses the getattr node injection. I actually like this, as adding state to function nodes was something to discourage to start with!

TODO:

• [x] Tests and docs for how this functionality works on composites

Contributes to #243

Closes #155

Non-goals:

• Purging SingleValue from the codebase

[github-actions[bot]]

:point_left: Launch a binder notebook on branch _pyiron/pyiron_workflow/automatic_singlevalue

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: +0.10% (target: -1.00%)	:white_check_mark: 97.62%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (b96ab3d86be163359f25faa570bc6c9db87b4715) | 3284 | 2870 | 87.39% | | | Head commit (b1ad266650e2d88dfc1cef7572bcd8975059ae27) | 3303 (+19) | 2890 (+20) | 87.50% (**+0.10%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#255) | 84 | 82 | **97.62%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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