[minor] Purge single value

[liamhuber]

Now that all nodes with a single output can function as "single value nodes", we can remove the class entirely.

Absolutely NOT backwards compatible, but a super easy update as one just needs to replace SingleValue with Function and/or single_value_node with function_node and everything should work fine.

Contributes to #243

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[github-actions[bot]]

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

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: -0.07% (target: -1.00%)	:white_check_mark: 70.31%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (c52cd1e8fcceb704031e8125fb77b2d0cbcc9a68) | 3303 | 2890 | 87.50% | | | Head commit (4f0812f357cdc44d36f9f5265c877894c74986c1) | 3284 (-19) | 2871 (-19) | 87.42% (**-0.07%**) | **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 (#257) | 64 | 45 | **70.31%** | **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%`

See your quality gate settings    Change summary preferences

_{You may notice some variations in coverage metrics with the latest Coverage engine update. For more details, visit the documentation}