Expose root node

[liamhuber]

Adds a new instance property Node.root: -> Node that returns the parent-most node in the node's graph. For the parent-most node or nodes all alone, the root is just self.

Access to this sort of graph property seems intrinsically nice to me, but in particular I have in mind to use it for specifying nodes as "checkpoints", i.e. points at which the graphs save themselves, by adding a finally clause to Node._finish_run like:

finally:
    if self.is_checkpoint:
        self.root.save()

I included documentation at the docstring level, but since we don't yet have a use-case (e.g. above) I don't see a need to modify any notebooks or train users on this property.

[github-actions]

👈 Launch a binder notebook on branch pyiron/pyiron_workflow/root_node

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.02% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (b26d224)	2572	2195	85.34%
Head commit (a08e11b)	2575 (+3)	2198 (+3)	85.36% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#165)	3	3	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines 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}

[github-actions]

Pull Request Test Coverage Report for Build 7576748024

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 90.246%

---

Totals
Change from base Build 7534444490:	-0.02%
Covered Lines:	4654
Relevant Lines:	5157

---

💛 - Coveralls